Over spinning when executing back kick is one of the biggest mistakes karateka make.
Here is a simple shotokan karate exercise to stop you over spiining on ushiro geri (Back Kick).
It’s very simple, pair up, then take turns to perform a spinning back kick, using your partner as a target. Try and stop on the target, then return to kamae, by moving back the same way as you travelled, so it’s like you just rewind back to kamae after the kick.
Try not to rest on your partner and hold for a few seconds, this will also improve your balance.