Right, I see some really good stuff here.

Nice job, everyone!
Let's see...
Not really killing is a good start but could take some work to ensure a permanent solution.
Helms of opposite alignment are good. An evil group I DM'ed for made some good use of these, coupled with some debuff spells to reduce the victims' saves (save DC 13 is not a lot). Of course,
miracle beats the
helm...
Taking them to Carceri is good. A group of high-level allies might be able to rescue them somehow but you can of course prevent that by having these allies meet a similar fate.
Turning them undead and having a Hunter of the Dead slay them is a very creative solution. I'll have to think about how I'd rule on that one.
Making good use of a plane where time runs faster is a cool idea but might not work.
True res works even without a body so faster decay probably shouldn't, by itself, count as a longer time dead IMO.
Making them enjoy afterlife so much that they won't come back is very cool from a role-playing point of view alone. It should work against all but the most loyal of people.
Changing the enemy's type so they can't be raised is a start, but higher-level magic can probably still bring them back. At least for outsiders; I'll need to look up some others.
Acquiring a poison similar to the Garrotte's would be a worthy quest.
Abusing ancient promises of the gods can certainly take care of this. Of course, figuring out how to make it work would probably require a
big quest. Not that that's a bad thing; I like quests.
Using a
wish to set up a curse that will hit anyone who brings the enemy back is very nice. Of course, it's most useful if the opposition doesn't have easy access to such magic as well.
Importing Keeper's Fang weapons from Eberron is an interesting thought. *researches it* Hey, these are
nice. I want one.
The Black Company ideas sound good. However, while the enemies might not be as powerful as those in that series, D&D still allows for magic like
true resurrection and
miracle, which will neatly foil most precautions.
Killing the deity is, of course, one of the best ways to ensure they can't bring their champion back. It's just that it's often a little impractical.

If you're very, very powerful, it might work against a demigod who has few other followers of great power. But otherwise...
Asking a deity opposed to him to lock him away can certainly work. Still, you'll need to get him to the deity's plane. Otherwise, stealing him back might well be within the reach of his allies.
Sanctifying the enemy is a good tactic. *pulls out BoED* Ah yes,
sanctify the wicked spell. Will save and SR apply. You'll also have to safeguard the gem for a year to make it work. Further, it costs the caster 1 character level. Still - nice. It's certainly worth a try.
Devouring his soul is cool but you'll need a creature who can do it.
Forging him into a weapon is a nice idea but it might be stolen.
Slaying all clerics works, but can be impractical.
Turning him into an undead works too, but you'll need to keep him that way.
I wouldn't disallow
true res just because the body is currently a
mindless undead. Ghoul +
bag of holding +
portable hole would work, of course.
Wishing away his name is very cool. It'd probably require divine intervention, though. Otherwise, even if it works, someone could
wish it back.
Dropping him on a plane with a faster time flow
alive is a nice plan.
Zombie +
awaken undead just might work too.
True, there's more out there than the power in the books. And yes, if they can't keep him dead, it's definitely a recurring villain.
The Soul Crusher can certainly solve this problem. It's not exactly my cup of tea but if all else fails it's of course worth looking into.
Thanks, all. You rock!
