Title says it all. I wish to "punish" a User who insists on challenging the Moderator through his avatars. Otherwise he is a longtime contributing User.

Can anyone think of a way to prevent him from uploading or showing his avatar?

You can make it for the whole group, not by member.

What I did when I wanted to prevent one member from doing something the other members did was to create a second Members groups called Members2 - move the offending member to that group and stop that whole group (him  ) from using avatars.