It is absolutely the responsibility of the indivdual. With that responsibility comes consequences. I like your take Simon that helping them to understand that they do have a choice and are making the inhumane choice to shun others is on them. Like corporations, religions are people. Until those people understand (by showing and demonstrating to them) that they are making the choice (whether they know it or not) to do something so uncaring, so unloving to their fellow human beings, they will think that they are doing right by god. We have the ability (not the responsibility) to help them understand what they are doing
My situation allows me to not partake in the shunning. I show up to family dinners, I work on my mother's house and stop in for coffee. I do not allow them to shun me. Nothing short of a restraining order or police forcibly removing me from the houses/families I grew up in/with will stop me from negating the will of my family to shun me.
I agree, show them they have the choice. Be relentless about it.
Good luck,
pbrow