Currently there is no functionality like that in the code. You could remove the leaving group capability and redirect the button to leave a moderator a message requesting removal from the group. That should work if your board is not too large.
Obviously, anything is possible with coding, as there is a moderation queue for accepting users into a group. Seems it would not be too difficult to add one for leaving. (Famous last words).
Wish I had better news for you. I can see the need for that myself.