Unfollowing a wave should remove it from the Inbox (currently it doesn't seem to). If the last follower unfollows a wave, it should be automatically moved to the trash (currently it needs a manual move). Moreover, the waves that live only as messages, don't have an obvious way to get trashed. That gradually turns the Inbox into a mess. So, I strongly suggest promoting this issue to High Priority.
I tried to delete a wave and I can't.
I don't know if it's posible, but anyway if it is and I can't find it then it's an usability problem.