I would just not give anyone but the FC Master the ability to promote other players to different ranks.
If you don't completely trust the person, don't give them access to promote.
However something SE could do, is put a cooldown on promotions. Once a promotion has been made, it takes 24 hours to come into effect, in the mean time the FC master or whoever has permission to promote, can revoke that promotion.
It won't solve all issues, but it will give people time to react to cases like this if they catch it.
