So its an Avatar you're applying to the player or is it an appearance they choose?
If its an avatar you're applying, you can probably just give them a bind to apply that custom appearance you want.
It's an avatar I am applying and forcing them to use it for the minigame. But they can technically choose and customize it with the bricks they select in the spawn room.That's the thing, no one is going to voluntarily change back when given that opportunity.The avatar selection needs to be forced on them.
You could always make it a bind that players commonly use, like crouch/jump, this way even if they don't do it voluntarily they can do it on accident.
That sounds kinda invasive and it will keep looping to the avatar change every time they press those buttons.