Soda does this, it's the most anooying thing ever!!
The trick is to be quick to pick up on when they're going to do it, get behind them with the lunge whip and send them forwards, it's a challenge to you so don't be afraid to get after them a bit (hard as that may sound)! I'm not talking beating them but give them a flick with the whip and tell them to bloody well get on with it!
The only thing you can do if it really is a straight spin in with no warning (Soda slows a teensy bit and slightly turns his head towards me when he's about to do it) is immediately, no fuss or bother, gather horse back up and send them off again like nothing has happened, try not to touch them or have any contact with them though, no talking, nothing. By not recognising the behaviour they're not getting any kind of attention as a reward (be it negative or positive attention) and they will realise eventually that even if they do spin in it won't make a difference they'll just have to keep working so they'll shut up, get on with it and hopefully the problem will get less and less.