For the curious, no, the frowned upon thing wasn't anything really and truly vile by most folks' standards -- just a thing that the particular sect is rather peculiar about.
Although the only big groups I can think of that do this are the Catholic Church with excommunication and Jehovah's Witness with defellowshipping (Judaism has something similar when you do something that renders you 'unclean', but their uncleanliness is a temporary thing, generally), I'm sure there must be other major religions out there that have this same practice (I'd offhandedly think the Amish would, should someone decide to embrace the Amish way after their 'wild oats sowing' passage, but then start playing with gameboys and wearing zippers). Now what I'm wondering is, wouldn't you expect there to be support groups for this?
Imagine you're a devout Catholic, you go out, do your Catholic things, tra la lalum, and *whoops* get excommunicated. Although the Holy See put a big red check next to your name, you yourself still are quite devoutly Catholic (sort of like the idea that Satan stays in Hell because God told him to go there, and Satan still loves God and follows his orders) even though you can't pop down to the local cathedral anymore. You'd think (well, I'd think) there'd be some sort of Deadbeat Club for such folks who still have faith in and want to practice Religion X, yet the local orders of Religion X consider you a pariah.