Yes they should be allowed. There is a spectrum, and it's going to be too hard (and unproductive) to write a policy that covers the mushy middle.
I expect we all agree that an answer advocating murdering somebody who ignored your "no solicitors" sign and wants to talk to you about Tupperware would be wholly inappropriate -- rude or abusive, actually. I hope -- but won't go so far as to say "expect" -- that we would all agree that an answer that advocates yelling at someone or slamming the door on the solicitor isn't inappropriate per site policy, though depending on the details it might be a bad answer deserving of downvotes.
Now what about the space in between? Let's talk about the mentally-disabled aggressive pest in a confined space from the question that prompted this meta question. Would you try to remove an answer that talks about self-defense techniques including force redirection? What about an answer that suggests the harassed women work together to physically push him out of their car? What about an answer that says to slap him in the face to get his attention? What about an answer that describes ways to use common items like umbrellas or house keys offensively without doing irrevocable damage? What about an answer that suggests threatening the attacker physically (but doesn't talk about actually doing violence)? And then we have the answer in question here, to use mace -- a remedy routinely suggested especially to women to deter violence aimed at them.
I didn't vote for the mace answer because it's a bad answer in this setting. (There could be other questions where it wouldn't be a bad answer, assuming the answer also explained why rather than just giving a one-liner like this one.) But I also didn't flag it as rude or abusive, because I don't think it crosses that line.
If we don't want this site to just be Nice IPS, then we have to recognize that sometimes answers might legitimately call for some amount of aggression, verbal or physical. Rather than trying to make a blanket policy, we should use our votes and flags to handle individual cases. Maybe when we have a larger body of examples we'll start to see a pattern that leads to a policy discussion, but I recommend against trying to shut down anything that smells of violence now. Once you're on that slippery slope, you're going to potentially throw away appropriate, arguably correct, answers some of the time. Either that, or you're going to have endless arguments about which gestures with what objects at what distance from somebody, or what volume or vocabulary in speech, constitutes "violence".