Lol, I have scores of bristle worms and I don't think that they have ever caused a problem in my tank," other than being ugly little beasties." Most of them are harmless as Dave said. As skippy said They aren't reef safe and will eat, fish, other crabs, shrimp etc. They will eat some bristle worms but, it's sort of a case of out of the frying pan and into the fire. The same with the hawk not only will it eat some of the worms maybe but, it will also eat other crustaceans like shrimp and small hermits.
The best way to control their population is to limit excess food in your tank. It's not as fact but it will work and generally increase the health of your tank/
hth
Chris