I personally have only been witness to one ban during my time there. There was a guy (I'm 99% sure he was an exceptional individual) who literally came into the store everyday and hung out. Every now and then he would buy stuff but he mostly came to just chill and chat with us. Dude made the mistake of doing this during holiday rush and was told by the manager, politely, that they couldn't chat and had to help people. Dude flipped his lid and began to tell people that Best buy and other stores had better deals while harassing the customers. My manager was so livid that he put dude in regional ban.
It really takes a lot to be banned though, the guy I mentioned above got mostly the banhammer due to him constantly bothering people. Majority of the time people are "flagged" in the network. Usually a flagged person is someone who will go to stores and trade over 200+ dollars worth of games for cash and do it again at another store. Or they buy high volumes of items and try to return them, etc.