My Son Makes Smacking Sounds When He Eats!

My son will be 6 in May. For the past three years, I've been trying to get him not to smack when he eats. The noise is annoying. I was taught at an early age to keep my lips together and don't remember what my mom did to teach me. I've tried everything I can think of to get my son to keep his lips together when he eats but he just gives me a blank stare and keeps smacking. I've tried punishing him, taking the food away for the night and putting him to bed hungry for smacking, I've tried bribing him with toys if he can go all day without smacking, I praise him on the few occasions he had eaten without smacking. Nothing works. He goes right back to smacking at the next meal. It's annoying. It's bad manners. Does anyone have any suggestions!!