It may help if you consider this site sort of like a church. While very few Christians would say an atheist is not welcome to attend their church, take part in Bible studies and other activities, there are obviously rules. (And any Christian, by the way, who doesn't want an atheist to come to church doesn't understand what it means to be Christian.)
For example, if you went to a Christian Bible study, you would have to assume that the people you're with love and respect Scripture. If you start arguing that the Bible is not worth reading, you'll be asked to leave, with good reason, right? If you're in the middle of a worship service, and you start debating with the preacher, no matter how polite you are, you're going to be shown the door. This should not surprise you.
I do not know the exact circumstances of your "ban" from Stilly. It obviously wasn't a permanent ban, because those are linked to your IP address, and unless you are using a different computer and server than you did before, the ban was lifted so you could be here again. It is possible your ban was unwarranted, but keep in mind that it is also possible you were saying something that really shouldn't be said within the context of a Christian site.
I personally welcome debate with non-believers. I think it's an opportunity for both of us to grow. I do see a lot of unbelievers, however, who have a kind of "poor persecuted me" attitude, and I don't like it on an atheist any more than I like it on a Christian.