I've been thinking about this. I think the best solution is to allow the deletion of a user, with all of the user's edits assigned to a single special user (say, "Trash" or "Junk" or whatever). That way, the references all over wiki would be intact, i.e. they would still point to somewhere (in this case, the special user) and there would be no need to rename dozens or hundreds of accounts to things like "renamed-spammer-1"
- the deletion script/specialpage would do the reassignment automagically.
Technically, that ought to work. Legally, however, you're asking for trouble. Unless contributions are made under the condition that all rights are released either to public domain or the owner of the site, then attribution is almost certainly required.