Aliia, unfortunately this is not the case, I had e-mail verification, user approval and watchdog all activated at all times. The spammers verified their e-mail, but I always deleted them instead of approving their accounts, so they had no chance to log in and add avatars.
I have not had any registrations since my last post, so I couldn't look into it further. One question though: does deleting a user account also delete any avatars they might have uploaded? Because I checked ow_userfiles/plugins/base/avatars and the smiley avatars weren't there.