At an NMAS show in San Diego, Luther played a song where it was solo acoustic. When he started playing, an over enthusiastic fan started clapping out a beat. Really loudly and noticeably, like he was trying to get the crowd to clap along. Instead, every one was really annoyed. While the glares didn't seem to stop him, Luther either sensed the crowd, or he was annoyed himself. Barely missing a note he turned to the clapper, smiled and made the "cut it" sign (ie., slicing your finger across your throat). The clapper stopped, and Luther gave a smile and thumbs up.
Classiest way to tell a fan to shut up!