So I’m wanting to do a few side plots for my players with vengeful or evil ghosts, but for the life of me I cannot find a single invocation or ritual that specifies them, only “tainted” beings (which I took to mean only spirits affected by the shadowlands taint). I know you can persuade spirits to leave the mortal plane once you basically resolve their “unfinished business” so to speak, but what if you can’t and they become violent? How do you get rid of regular untainted ghosts?

You might want to check again because the quality or trait you are looking for is "Otherworldly", and quite a few spells and rituals can target that one (including all the classics, Jade Strike, Bind The Shadow, Tomb of Jade)

Thank you for the clarification, because I wasn’t sure if “Otherworldly” applied to normal human spirits, but in retrospect that was probably dumb on my part.