Businesses have the right to deny service to anyone they like at any time, as long as the reason wasn't based on the other person's race, religion, gender, or sexual orientation. It's a version of the 'bad tie rule', which goes something like this:
"You're wearing a bad tie. Get the hell out of my store."
In other words, they can't keep you from talking about computers, but they can always ask you to leave their store. That aside, unless you two were very loud, that's a pretty stupid reason for a businessman to kick out a customer.
Now, Hippypancake, to play devil's advocate, although that player had absolutely no right to toss that miniature across the room, that doesn't give you the right to sock him in the nose. Complain to the store owner, toss his stuff around too, or even call the police (although they might just seem amused by the situation, honestly). Escalating directly to violence is a good way to wind up in prison.