I had my preferences set up to block NPB's, but removed them several weeks ago.  Was shocked when I had a look in the log details of just how many people tried to purchase in recent months and were stopped due to my blocks (7 pages full!).  Some contacted me, and I was able to exempt them, and I have not had a problem with any so far. So I am more inclined nowadays to allow anyone the opportunity to purchase, irrespective of what's happened in their history - DEFINITELY MORE SALES $$$:-x