The Spell Compendium contains 2 spells I simply won't allow in my game:
1) ironguard, which essentially makes a character completely invincible in many situations. Perhaps I would allow it with a level-based limitation on the enhancement bonuses of magical metals affected (+1/four levels, max +5 or something).
2) greater consumptive field. With Persistent Spell (or even Extend Spell), an evil epic cleric could simply teleport (with or without help) to a village or small town, walk around a little bit, watch everyone drop dead, and gain hundreds or thousands of points of Strength and ridiculous amounts of temporary hit points. He or she could then teleport (or better yet, step through a mirror of mental prowess) into the vicinity of an extremely powerful creature and kill it in one round.