I'm reasonably comfortable with moral grey areas, no halo, but dragging some poor sap out of the pub under false pretences and subjecting them to an abominable ritual sacrifice is too rich for my blood I'm afraid.

I've been reading that you can't get rid of this ghastly mission either by simply refusing it or by killing the cult members by the shrine. This true? Is there perhaps at some point in the game an unmitigated scumbag that's going to get it anyways i could use for this?

Any other ideas to get rid of this quest?

U cant cancel the quest, once its started! Even if u kill the cultists u still have to do the pillar thing!
Personally i took an orc from a stronghold and sacrificed him!
There may be another way i think!
If u have dawnguard DLC, there are some radiant quests about a vampire in disquise. Once u find him u have the option to follow u. U can try to lure him to the pillar and see what happens but i doubt it, since u must command him to go to the pillar!

I only did this quest on one character that did all the daedric quests. I used the cannibal lady from Namira's quest.

If you're on PC you might be able to setstage this quest to completion.