It's not quite an argumentum ad baculum, but what I've come to call the "punch in the face" argument against moral relativism and nihilism, sometimes sounds like one.
I was once attending a philosophy seminar on Plato's Republic, which I have to say, I honestly and thoroughly enjoyed. The professor was someone I very much respect and admire, and I generally like his style of teaching philosophy. But one day, while discussing moral relativism, the professor (who, I'm fairly sure, is mostly unsympathetic to the idea, and leans towards moral objectivism) said something to the tune of: "How do you debate with someone who claims that morality is relative? You punch him in the face!" Given the context, it was clear that the argument is supposed to apply not only to relativists, but also to those who deny any sort of truth (relative or objective) to moral claims or existence to moral facts and properties (i.e. nihilists).
Supposedly, the argument is intended to show an inconsistency in the beliefs of the relativist or nihilist, or at any rate a mismatch between what he or she claims to believe and how he or she actually lives. The argument tries to demonstrate that without belief in moral truth or actually-existing moral properties and entities, a person has no satisfactory way of responding to a punch in the face, but just has to accept it. Clearly, the person can't respond by saying "That was just wrong!" or "That was immoral!" without contradicting themselves.
I've seen the argument pop up in different guises elsewhere. While perusing an online forum dedicated to Ayn Rand's Objectivism, I came across a post from an individual who very clearly wanted to be a die-hard, Objectivist "True Believer," but was experiencing some cognitive dissonance as a result of a discussion with a self-described moral nihilist. Basically, the person who posted was asking his fellow Objectivists if they could think of a way to successfully and convincingly refute moral nihilism. He had tried to do so by presenting the nihilist with a form of the "punch in the face" argument, asking how he would respond if some unspeakable cruelty were committed against him or a loved one. Apparently he did not get a satisfactory answer from the nihilist, who sort of danced around the question.
An even more straightforward form of the "punch in the face" argument simply states that the moral nihilist cannot morally justify his or her own existence, and therefore it's fine to, say, kill moral nihilists. Or, at the very least, they can't reasonably have any objections when you try to kill them. The relativist, likewise, can only offer a relative, limited, let's say "half-assed" moral justification of his or her own existence, and so it's apparently fine to kill them, too.
This is a dumb argument. In my view, it's like trying to refute atheism by arguing that a person who doesn't believe in God has no one to answer his prayers. Well, an atheist doesn't believe in a God, so praying to one is probably just something he simply doesn't do. Likewise, someone who doesn't believe in objective moral truth isn't going to bother with trying to offer moral justifications of their own existence. But to assume that, even so, a moral justification of one's own existence is required, is more or less to arbitrarily presuppose some kind of moral realism. That is, before you go on about your business existing, first you have to justify it by logically tracing your continued existence back to the solid foundation of some moral fact or truth. I think that's very silly.
Likewise, if you're a moral realist and/or objectivist debating a nihilist, and you declare, in Samuel Johnson fashion, "I refute it thus!" and proceed to punch him or her in the face, or visit some unspeakable cruelty upon him or her, you've proven nothing about the reality or truth of morality. The nihilist doesn't need to say or show that what you've done is "wrong," but can safely state that what you've done is contrary to their preferences and can either respond in kind or, perhaps depending on the nature of the unspeakable cruelty, simply die laughing of your pitiful attempts to prove a point.
One doesn't need to offer complex metaphysical or epistemological justifications for one's own preferences and desires, and a moral nihilist doesn't necessarily deny that people have preferences and desires. Your preferences, desires, values, and inclinations are simply part of you, requiring no more philosophical justification than does your eye or hair color. They are the result of your general physical and psychological constitution, which in turn is the result of the interplay of a variety of environmental, biographical, genetic, historical, and cultural factors. I don't need to write a philosophical treatise for the cashier at the corner store when I choose Pepsi over Coke. Similarly, I don't need to explain why it's "morally justified" for me to go on living, or to not get punched in the face. I'd simply like to go on living, preferably without getting punched in the face, and if you have a problem with that, then let's talk about it, or if you aren't willing to talk about it, I suppose I'll either have to, you know, call the cops or otherwise resort to force.
One does not need to prove the mind-independent existence of a chair in order to sit in it. Likewise, nihilists do not need to prove the existence of a sturdy, iron bar in order to pick it up and beat the living poop out of those who punch them in the face in order to prove a philosophical point!
As I've hinted at previously, I make a distinction between ethics and morality, and in particular I feel that ethical inquiry is hindered by the assumption that we are constrained by moral facts, laws, and truths. There is no morality, no right or wrong. There are only consequences. Ethics, as I understand it, is primarily an exploration of consequences, or thinking and talking about which thoughts and actions lead to, or rule out, which practical results. That is all. There are better and worse ways of achieving stipulated goals; if you want to cut your hair, and you are given the choice between using scissors, or using a grapefruit, it's probably best to use the scissors, unless your desire for citric acid in your eyes outweighs your desire to cut your hair.
That said, I feel that ethics is the heart of philosophy, and ethical inquiry is what renders all other forms of inquiry meaningful and relevant. Scientific, ontological, and epistemological questions are pointless unless they orbit around ethical concerns; a map is useless without a path and a destination. The abandonment of objective morality did not lead Nietzsche, for example, to stop talking about how we should live. Ethics, in the broad sense I defend here, involves talking and thinking about how to live, and is always an open question, not admitting of final, static, indubitable answers, but is perpetually open to the flux of life.
I suspect that the "punch in the face" argument rests on the metaphysical prejudice that reason is prior to and determines passion or desire, and that therefore our preferences require rational justification, just as our knowledge claims require some evidential or logical justification. But I agree with Hume's observation that reason is "the slave of the passions," and Schopenhauer's argument that will is prior to intellect. Practical reason is concerned with the adjustment of means to ends, but the question of ends is not amenable to reason. No cold, impartial, objective assessment of reality will tell us what ends we ought to pursue. Reason or intellect is the sail, and passion or will is the wind.
This may be an uncomfortable conclusion for many, because it seems to entail that (given the wide variety of preferences and desires of different humans) there may be insoluble ethical disagreements, incommensurable accounts of "the good," and so potentially endless conflict, chaos, and dispute. My only hope is that discussion and compromise can take care of most of this, and maybe we can keep the punching to a minimum.