I'm not sure about the legalities. It sounds like he's just begging for trouble and a gigantic PR nightmare for his business, though. (On the other hand, people are funny. Maybe this will serve as a lightning rod to attract other zealots of whatever creed he follows, and they'll all congregate there.)
If that's the primary avenue for support, and the person is effectively saying that paying customers of theirs cannot use those forums because of their religion...then I think there's legal ramifications involved and the person has done something very, very foolish. If the forum is a community but not firmly attached to the business, and/but the people he's banning are not paying customers...the case against the person's actions may be harder to build.
Sounds a bit slimy, from what you've posted. I can see banning people who are actively abusive to sysadmins or other forum personnel, or trolling/flaming, or promoting illegal activities like theft, violent protests. But even if someone's discussing something wildly off-topic, if they're not being abusive or promoting illegal activities, it's kind of difficult to find a cogent reason to ban them.
Contact groups specializing in cases of religious discrimination, and find out how familiar they are with online laws and discrimination. That would be my first step. Print out and/or get screenshots of any post you think may support your case - because if this person's banning people of a certain religion for no reason other than their religion, they may start pruning forum content if they get word that someone's going to push back.