Forgive me, but this seems like a somewhat silly question. Angels, whether fallen or not, are creatures of pure spirit. Death, as we temporal human beings understand it, doesn’t really apply to such beings. Truth be told, it barely applies to us. Every human being ever born is immortal, in the sense that we will truly exist forever. The only real question is where…
Along those lines, and pardon the lack of scriptural citation (such things have been covered endlessly for centuries by minds far more brilliant than myself), angels and humans both have the distinct privilege and responsibility, as beings created with the priceless and perilous gift of free will, of choosing the path of ultimate life or death. Fiction often portrays angels, both fallen and ascended, as strictly temporal and mortal beings, subject to injury and even death in the endless war between good and evil.
For the purpose of telling a story, much can be ceded to artistic license. But in reality, I am convinced that angels, as sentient beings of pure spirit, are just as ultimately immortal as we humans are, and would therefore be subject to similar (though apparently not identical) rewards and punishments for their actions. The question of whether God could destroy any human or angel if He so wished is immaterial. Of course He could, He’s God! But so far, He has clearly seen fit to NOT destroy the angels that fell, despite the fact that our meager human existence might be quite a bit easier if He had.
That being said, God has given us a rather wide variety of spiritual weapons with which to combat demonic forces. Destroying them outright doesn’t seem to be an option within our mortal capabilities, but Jesus Himself demonstrated for us that demons are quite vulnerable, helpless even, against the tremendous power of faith and love. Demons themselves are no longer capable of either, having lost all hope after their fall. That same gift that they were offered but rejected, which we are now called upon to accept… that is their undoing. And I dare say, mere death would be a tremendous mercy, if only they deserved it.
As for the angels that remained faithful to God, it would appear that they have already been glorified, thereby becoming immune to any form of death or destruction that we humans could ever comprehend, let alone possibly inflict on such beings. In short, is it possible to fight and even defeat a demon? Absolutely. But is it possible that such a being can actually be killed or destroyed? I dare say, no, it is not.