you cant get rid of bristleworms without nuking the whole tank and staring over. And even if you did, theyd come back on the next piece of live rock, macro, or frag anyway.
But that doesnt excuse use from exercising correct logic. The often repeated statement that theyre beneficial or harmless is utter nonsense, since all tanks have them and we cant really compare a tank with and without them. The statement that they only eat things that are sick or dying is likewise nonsense, since its not falsifiable as written. Any time you report bristleworms attacking an animal (which I've seen many times) someone will claim "thats because that animal was actually sick anyway".
People who exercise such poor logic should not be listened to on any subject.
The most accurate statement about bristleworms is "some people consider them harmful, and some beneficial, but you cant really get rid of them anyway so probably best to accept their presence".