"This is, of course, the most pervasive problem in D&D, and one which the rules have never addressed. The tension of many battles has been ruined by some smart-alec suggesting they use the Holy Hand Grenade. No fortress – no matter how impressive or dangerous – will ever seem foreboding after one of the players points out that, “It’s only a model”.
My own suggestion for the 4.0 edition rules: Anyone who quotes Holy Grail during a session should be made to eat their own character sheet."~Shamus