you could show Bobs messages greyed out, and your messages are greyed out for him. Or with threaded conversations, you prune Bobs messages and all replies. It

you could show Bobs messages greyed out, and your messages are greyed out for him. Or with threaded conversations, you prune Bobs messages and all replies. It might be slightly confusing, but it's better than having to leave a group entirely, or see their abusive messages, when someone is harassing you.