The advice in the subject line is correct. Users
should never be
deleted from the table for another reason; you leave a lot of messy
references to them lurking about which really does confuse the hell
out of the software.
MediaWiki does not support the deletion of users, and likely never will.
But, how should we react, when there is a user, who _only_ uses his account
to put Spam into the wiki? - Is it reasonable not to delete such a user?