How about a mod disabling all avatar changes that don't require the GUI.
This is not possible. You clearly do not understand how Blockland works.
Server-sided mods CANNOT tell what a client is doing beyond what the client tells them.
As it so happens, this includes avatar changes. There is only one way to change your avatar, so all avatar-changing mods, by necessity, will use this one way. This also happens to be the same way that the Avatar GUI uses to change the avatar.