The posts here will help clarify some:
But I have to agree, it might be nice for the mods keep a list in there about who's taboo. Since signing up a couple months ago I've felt a bit like it's a mine field too. The reason they were banned doesn't need to be stated, just that we don't discuss them here. I think it might be especially helpful for people who've done their shopping and searching before joining HP and so they might not realize that the vendor they've been dealing with isn't discussed here.
Or maybe a statement somewhere that says we only discuss vendors that ARE on a such a list.
I dunno, just a thought.