It's not just bicycle swimmers - many photographers are guilty of not watching their feet or where they are cramming their strobes, concentrating only on the object in their viewfinders. Reef molestation is an offense is committed by both newbies and veterans alike, and while it's more understandable in less seasoned divers, it should be addressed in either instance.
I, too, have done the scream through the regulator, and often clasp my head with both hands when it's really serious (international signal for "omigod!"), but I do have a more effective tactic which I usually employ. I gently grab the diver's fin, move it away from the subject of it's attack, and hold on to it. When they try to swim off, they feel resistance and turn around with a surprised "what the ?" look. I then point to what they were just kicking, and they immediately get the message. Usually, nothing more needs to be said, and it eliminates any possibility of denial or hard feelings later. People aren't nearly as sensitive or defensive when they're caught in the act, and tend to think more about what they're doing when they know they're being watched.
Pet peeve #2 would be those who pee in their wetsuits, and then want to stand right next to you after the dive and have a conversation. To all who are guilty, please know that WE CAN SMELL YOU, even though you can't smell yourself. It's much worse than farting, which most people would avoid subjecting you to. I never know what to do in this situation, short of fainting. Any suggestions?