Are your players running roughshod over your monsters and ransacking your dungeons? Are they disrespecting you as a Dungeon Keeper?
DungeonCo here to help, with the handy dandy Wings of Wondrous Power
A possible answer to what happens to spells when a mage dies. If the spell is strong enough, say and enchantment or other permenant effect, part of the mages spirit may become lodged in the magic. It may be a way for items to gain some kind of intelligence, but a mage who has knowledge of this fact would be very hesitant about enchanting anyone or thing. He might have other plans for his afterlife than counting the change in your bag of holding.
Preists, I think, would have this sort of thing covered.