A common LegalSolution is a "no offensive usernames" policy, as used on EnglishWikipedia. The problem is that it's hard to define "offensive", and people online come from many different cultures and traditions and ways of thinking. For example, the US has a stronger tradition of unrestrained FreeSpeech than many countries. Of course, once something is written down, some RulesLawyer? will come along and try to find cracks in it. AntiAuthoritarians don't make things any easier.
Almost invariably, offensive names are WalledIdentitys. Thus, a policy of requiring either RealNames or longstanding PenNames is a useful workaround, but this may be too big a step for some communities.
Interestingly real names can be offensive too. In the Olympics the English rider "Pippa" has been renamed "Phillipa" incorrectly by the Greek media because Pippa is apparently very rude in Greek. Similarly President Putin is called Putine in France.--AndrewCates
There wasn't a policy beforehand, and I don't think it was even discussed. Presumably everyone felt it was one of those things that goes without saying. (cf. BehavioralNorms)
That person isn't really forcing the issue. They're just being childish and really not particularly clever. It's clear that anyone can create a "UserName" that is offensive, just like anyone can continuously write profanities on the site. It's not an amazing achievement to recognize this. Indeed, we AvoidIllusion to LimitTemptation to do this sort of thing. So, ask yourselves what do you normally do in a social situation when someone, say, starts making fart noises? Glare at them like they are idiots and then turn your backs to them. It's not wrong to shun annoying pests from your online community, although also be careful to avoid the compsci tendency to maximally include, but permanently exclude.
It's a particularly cryptonautic thing to formulate policies (a LegalSolution) for every possible situation, but in general you should just rely on your innate social skills, which are far more adaptable and tolerant and powerful than any written policy could be. That is, a CommunitySolution. -- SunirShah
Another interesting case yet to be digested is how a potential intellectual property donor pulled back from Wikipedia after seeing the nick CrucifiedChrist? posting. He decided Wikipedia wasn't a serious attempt at an encyclopedia. 
Much of this discussion is specific to the situation with the MediaWiki software which ascribes greater significance to user names that UseMod. A MediaWiki user cannot change their name without losing their sysop status (if they have it), their watchlist, and their list of contributions. All these capabilities are, for at least some users, critical to the Wikipedia experience.
A subsequent case at Wikipedia involved a username "Jesus Is Lord!". The username was, apparently, nothing more than an effort to provoke conflict, in which regard it was successful; also, it was revealing of a degree of anti-Christian bias among the contributors.