They should. If the customer is no longer interested, and expressed it clearly by asking to be placed to DNC list, they should respect it. There's a lot of software out there that makes it as easy as clicking one button, or even easier - you open your CRM software and see big red "ASKED TO BE PLACED ON DNC" all over the record, and you know it didn't work out, time to move on. Of course, some salesmen may ignore that hoping they can persuade the client to reconsider, but then that's where $500 comes into play.