I do know of some admins who doesn't allow their users to upload avatars...Only the admins can. Basically they lock the avatar option as the default setting but when a user requests an avatar the admin will place it in for the user, then re-locks the avatar option.
For myself I went one step further. I used an older version of Qasic's Avatar Hack (w/addon) so I could enable Flash. Then I stripped away the options in the registry and profile fields where the avatar text areas would be. Instead, I wrote the instructions for users who wish to have a custom avatar. Now in the users' profile if they do have an avatar in place it will display at the right column. If a member has an avatar, I'll upload it for them.
The only avatar option I know is the group settings (e.g., admin, member, jr. member). If I read you correctly you want a flexible setting that allows you to chose what individuals can have avatars. I do not think there is a hack for that yet (I could be wrong). Currently the best option would be the one I explained above.
