I recently made a hack to make a user's avatar invisible to the rest of the users.
The user won't know it, other users will not see his avatar, and the mods and the admins will see his avatar and a red writing below it that says
Avatar Offender.
I have modified the admin control panel to let the admin turn avatars on or off, and modified the admin CP search to allow you to list users with invisible avatars.
If this interests you, I will publish it.