Unless it's someone in your lobby whom you trust, do not hold off on banning an OP champion in hopes that whoever has them declared will carry. 11 times out of 10 they will feed if they get the champion. 12 times out of 10, the enemy will stomp you if they pick first.
The same goes for trading a strong champion to someone random.
And if they decide to feed for it, then there's good news. The punishment system is working as intended.
This is just to protect you from having a bad time. This is a GAME, after all. You've no obligation to take the risk to your fun.