My members who signed up will be placed in a usergroup "Member", here the members cannot upload avatars.
Until they posted 15 messages, they will be promoted to another usergroup "Advanced Member", here they can upload avatars.
But there are somehow a few people get through and able to have their own avatars even they have only zero message posted.
I did not allow Facebook signup.
Is it due to the gravatar? But I have already checked, they do not have the option to change Avatar under their personal details area, so what's more choosing the option gravatar?