“Impostor” literally means a spirit that is pretending to be another spirit. This was just a vampiric parasite, aka “feeder” not an impostor.
(I’m being particular because language matters when you’re trying to explain what’s what and we have a lot of newbies here lately. )
You can kill them yourself - no need to rely on others unless you enjoy doing it that way. Envision a sword or something and get personal with it.
I like to wrap them up in black and lob them through the black hole in the center of the galaxy, which banishes that pattern from this universe, aka “killing” them for our intents and purposes. The pattern can rekindle in a different universe.
Note: Death is not a real thing, it’s a door - they’ll only lose themselves if the want to. Perk of this planet’s energy.