Adding more protein will only really help if your birds are craving protein. Most likely, however, they are doing this out of bordem and habit. Your best chance is to cull the peckers or separate them. Prune the dying plant and save what can be managed, then let it grow again. I've dealt with this all before. I know how frustrating it can be.
Sunflower seeds have lots of protein. For a flock that is experiencing deprivation, it's a perfect treat and will usually halt pecking, because sometimes pecking occurs from lack of protein that the birds want. (Blood and feathers contain protein.)