You need to separate them firstly. If it is the DP birds going after the broilers and not a predator, they're able to do it because the broilers just aren't going to get up and run or fight back, they basically want to sit around and eat. I would hazard a guess that space is a big factor, probably did not have enough. At this point they may not stop no matter what you do, so you need to just give the broilers their own safe place. Not feeding enough protein can promote feather eating/plucking in chickens, which could end up going farther. It's really hard to tell online what the cause may be, but if it really is the other birds doing it, someone's gotta move.