I have definitely done this. I have little stickies on the front of my computer that lists all whom I will not buy from.
This brings a question to mind. I know how to block a bidder on ebay, but is there a way to block me from bidding on a particlar seller. My lists is getting longer and sometimes the seller lists some good chips. I still want to denied my patronage after being taken once. Being taken once, shame on you; being taken twice, shame on me.