Since it's been mentioned a few times, a quick lesson in Psychology terms:
1. Positive reinforcement: you do something good. you get something good.
2. Negative reinforcement: you do something good. you lose something bad
Reinforcement is when you do something good, you get reinforced.
3. Positive punishment: you do something bad, you GET something bad
4. Negative punishment: you do something bad, you LOSE something good.
Punishment is when you do something bad, you get punished.
(lots of people frequently call negative reinforcement punishment incorrectly)
EXAMPLES of each:
1. In a session with no pop references, you award the whole group (or individuals) bonus xp.
2. In a session with no pop references, you turn off the annoying buzzer that you play for every session. "Ya want the buzzer to go away? Stop annoying me."
3. They make a reference, you summon a monster who kills them.
4. They make a reference, you dock them xp.
Umbran is correct, though, that changing behavior is best done through positive reinforcement (ESPECIALLY in a gaming group of friends looking to have fun).