Keeping the dead... dead.

[Viktyr Gehrig]

Animate the corpse and throw it off the edge of Sigil. If I recall correctly, not even wish level magic can return someone or something from that particular Void.

Styxing him before killing him should help, too-- it'll make him less likely to willingly accept resurrection from his allies. If you can catch him alive and mostly incapacitated, I think you can use mindwipe and psychic chirurgery to make him irreparably and utterly useless to his allies-- no sense bringing him back if he doesn't remember anything and can't do anything.

 

[Iku Rex]

Dunno, if it has been mentioned, I noticed a special material for weapons in CW, which allows to trap the soul of a slain victim.

You beat me to it.

"Thinaun", Complete Warrior p 136, traps the soul of any creature touching the weapon when the creature dies. The creature can't be raised/resurrected in any way unless you have the weapon nearby. (In the latter case you only pay half the material component for the spell.)

 

[comareddin]

Reincarnate him as a frog and carry him in your pocket?

Com

 

[moritheil]

The fact that a spell to keep evil souls in the afterlife is not a fundamental one for divine magic is very odd. Even given that the spells in D&D are largely about combat, and don’t represent at all well the magic a society as a whole would try to discover, the lack of this power is a glaring omission, I think.

This is not odd in the least with respect to divine magic. With respect to arcane magic, it IS a bit odd, but if you were a deity and one of the main draws of your faith was the ability to bring anyone back from the dead, wouldn't you want to stomp on this up-and-coming mage who was researching a way to undermine all that? It would be an issue of personal pride as well as business.

 

[Elder-Basilisk]

If he's not a spellcaster, baleful polymorph him into a toad and carry him around in a cage.

Otherwise, the imprisonment and binding spells are made for this kind of thing.