Positive reinforcement is much more powerful then negative reinforcement, but will begin to addres the bolting issue only when used correctly. The horse is already bolting and horses generally don't go from good behavior that warrants positive reinforcement to bolting without quite a few steps in between - - small little behaviors yes, but still present and signalling what's to come. It's not like they were out for a relaxing ride and a deer flew out of the woods and startled the horse and it bolted in panic. But whatever, praise away and cross your fingers that it's actually rewarding the right thing.