I think this is a good idea. A lot of people spam whisper in large servers because the ops won't know about it and wouldn't have suitable proof of them spamming, allowing people to stop other's whispers would help prevent this. Also just being able to ignore someone through chat commands would be nice, annoying people might not be so problematic and the op wouldn't be asked to mute them (which tends to cause a lot of rage from the annoying person).
Good idea.