There's two possibilities with being banned after the beta: 1) They were reported during the beta and the case didn't reach investigation until after it ended. The person still did something wrong though. They likely would have been banned after their purchase regardless, but at least Battle.net is giving out refunds from what I hear. 2) They had something considered malicious that was caught by automation when they first logged in after their purchase. Which again, can't be warned of in advance without any or all of the 4 retailers performing some sort of desktop scan for banned things at checkout. No matter what there are no systems in place on any of these retail sites to catch bannable violations exist OUTSIDE of the game before a person makes a purchase and then later triggers it. People banned might be sent an email, but I don't know.