"You have to stop. It's not funny." (well maybe a little, but I certainly can't laugh). "If you say 'poopy' again you'll have to go to your room."
"Oookaay" says my 3 year old with an enormous smile on his face. As he walks away I could swear I hear a whispered 'poopy' but I can't be sure. Do I continue this inane conversation or do I just let it go for now? I decide to drop it. I'm sure it will come up again. And it has.