Bristle worms are beneficial poop / extra food eaters and are great to have. They will only multiply as large as their food supply, i.e. the more food you feed, the more fish you have, the more worms you'll get. That to the side, if your are dead set on trying to get rid of them, rinse the sand like stated above. Besides, depending on how you took the sand out and how long it was in the tank to begin with, you could have another cycle if you don't rinse.
88g L shaped reef
150g freshwater cichlid