See, if I was a GM, I wouldn't necessarily disallow Pun Pun. I'd just reason that, if Pun Pun was possible, in all the multiverses of all the previous history of D&D someone would have happened upon it at random. Therefor, there is all ready a Pun Pun, who isn't really bound to any universe due to his enormous abilities. This Pun Pun could easily achieve total cosmic omniscience and thus instantly know when someone else was entering the path to become Pun Pun. So the second you get that Candle of Invocation and start summoning Pazuzu, you are WTFpwned by a force that makes even Immortal Handbook creatures quiver.
Hell, Pun Pun probably just killed off all the creatures with Manipulate Form and Pazuzu for good measure. Could genocide the efreeti while he was at it.