Use a little humour to defuse the situation. I know that players feed off the emotion of a slam, but in going for it, they sometimes kill that emotion with a miss of an easy bucket. You know the player feels embarrassed, so you suggest that it would have been better if he has settled for a lay in instead of a dunk. They both count two points, so what's the difference?
That's the difference between a pro who gets the job done no matter what, vs. a player who does it thinking style over substance. You've get to get them to think like pro's.