Zagreus,
This is one situation where "Just say NO!" is definitely applicable. There's a difference between being receptive and being passive. In order for the process to work, you DO NOT have to be in such a totally passive or "mediumistic" state that you entertain anything that comes along regardless of whether it is good or evil. If you sense evil...STOP!!! Just stop channeling and go wash the dishes or something.
Another thing: A "good" entity will never attempt to interfere with your free will. Mine have even told me they can't or won't do that, much as they might like to at times and as much trouble as it would save them in the mopping-up department.
Also, there's a limit to what they can do where karma is concerned. They can mitigate the natural consequences of your actions but can't nullify them completely. I guess it's obvious that I only have experience with protective spirits, although I understand there are other kinds.
--Linda