Once you add a user, there is no way to delete them via the Wiki structure (I assume there's some way to do it via SQL, but maybe not if using sqlite.) There doesn't even seem to be a way for admin to disable them, such as leaving them in the database but preventing them from logging in.
Let's have a discussion on what "deleting a user" really means... I've dived a little into the script/util/delete_inactive_users.pl program, finding that it tries to delete stuff, but (as of this writing) might fail for some reason.
Before this can be solved, there's at least one question that needs to be answered – If a user account is deleted, what should happen to the page_version entries linked to that user? (atm, they're deleted /with/ a cascade-on-delete to it's parent page_version, recursively deleting it all).
There are some options:
Are there any other options?