I abhor that underhand practice, and can't think of any reason why it is valid. If you want to tell a user to 'go away, leave me alone', you should be able to block them without having to leave your groups - it doesn't need to be secretive.
That’s reasonable for you?
I mean it’s ok if you want to just say what they’re doing sucks. But I’d find your position more compelling if you could offer a reasonable alternative.
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.