Most people who don't like what a presenter is saying walk out of the session. Some may be vocal and boo or hiss (do people still hiss?). Or maybe if the session is great, people will clap or say woot-woot.
Those are time-honored traditions. However, for those seeking something more up-to-date, check out LVnH8, a flashcard system for your portable devices. Just pick one of the 300 cards and flash your message to whomever you'd like in the room.
Flashcard sender beware: We make no promises about how the receiver of your message will react.