First of all there is no possibility to disable all avatars like it was on phpBB2.

Second when you disable for example external avatars or upload avatars the avatars which were already set are still shown and when the admin disables all three possibilities the user has no chance to delete his avatar.

I wrote a fix that adds a "Enable Avatars" at the ACP and stops showing the type of avatars which are disabled on the viewtopic and profile page. I sent it directly to nickvergessen so that he can review and commit it.