Well, first, Santa is not affiliated with any one religion, he is just affiliated with this particular holiday, so it isn't possible for him to be Satan. Second, his positive actions would fly in the face of everything that Satan would supposedly want.
I can imagine that you might yourself on his bad side if you suggest that he is the epitome of all evil though. I can't imagine him appreciating that, especially after all he does to make your holiday special year after year, for no pay. Seriously the guy is an example for all charitable organizations, which is another reason why he can't be Satan... I can't see Satan doing anything charitable.