However, my current one demon/devil lords have 40-80 HD. My demi-gods have 40-60, so in sense of combat statics, demon/devil lords are around same level than most of demi- or lesser-gods. Good beings, and elemental & other planar lords are about on same level, but I speak of devils/demons for short. Mostly because they are something pc:s of mine most often want to kill. Their earthy manifestions are often less poweful, so 18-20th lv party with right tools have possiblity to melee-banish such beings.
Though actual problems come from insolent demi-gods, lords of evil and innocently destructive elemental lords, and some alien things with reality twisting presence. For example.
My gods aren't worship dependant, though it adds something to their power and more to local infulence in worlds of mortals, and they just plain like it.
For demon/devil lords mere worship don't add to their power, but it grants them certain freedom and access to world of mortals. And some of them also like worhip, though not all. So stronger cult-fallowing they have, the more likely it is they are able to manifest without help, though typically in less powerful form, or through possession. They also need sacrifises, simple ""praying" won't do the trick.
Anything more than demi-god is a concept, and such beings can't be "killed" actually in simple combat. With special means you can kill gods, which means killing part of world or at least some spiritual concept behind some part of nature.
Demon/devil lords aren't so important personally, but they are part of primal concept. If they truly die on their own home turf they die, but new bearer for that power is going to take the place sooner or later. Under different name, though. Dead demon/devil lords can also be brought back with some special means, as well as demi-gods.
Everthing in this cosmos of mine can change or die. Planes can vanish, worlds can die, universe itself can be annihilated. But killing or changing powers that manifest those things and that are manifested in those things is not an easy task. Doable yes. If pc:s want to take that as their personal quest, I sure am willing to create that adventure. With lot and lot of twists.
Gods don't kick demon/devil lords, for these reasons.
a) "Evil" gods like them as power, though have killed in past some personalities they despised
b) Others than evil gods can't manifest on homeplanes of lords of evil, not with their great power anyway
c) Just killing them would't help much, except maybe sometimes settle personality issues
d) balance and certain free-will for mortals reason when those powers manifest on mortal plane
e) on certain concept level they are killing them, all the time, and they are constantly regenerating
f) But they do, demi-gods have killed them and vice verse
G) where comes in: Maybe pc:s are heroes than can with their deed change something to break the cycle
I don't have inspiration now to make real write-up about this. But something there is. 
I don't generally pay too much attenton to WotC epic or divane rules. I use those bits that are cool for me, and give kick to everything else. i also don't mix Planescape with my d&d, it doesn't fit.
I also don't ever use idea of worship dependant gods btw, unless it's Discworld game.
Or HOL.
I instead use vampiric alien entities who sometimes play gods. Stargate Orai (or whatever they were called) has some similarity to that. Sure players don't always know the truth about divane/other entities.
