easy enough: develop a knee-jerk, and anti-PC, reply to all such unsolicited mailings. The amount of abuse to be used for seasoning is to your taste.
This happened to me last month. I advertised my boat in Boat Shopper. I got an email saying this person wanted the boat; he came in $200 under my offer, so I jumped at it. Then he said that his principal (he was an agent) had planned to buy a boat, and had already cut cashiers check for $21,800. I had agreed to $14,800. The guy said that when I got the cashiers check for $21,800, I'd write him a check for $7,000. Immediately the warning flags went up and I emailed him back saying that the deal was off and not to send me anything.
Ten days later, I got a cashiers check in the mail, sent from England. I opened the envelope and found a cashiers check for $38,000. I notified the USPS Fraud Department and my local police department. That was the last I heard from those weasels. The cashiers check is still sitting in my desk, unclaimed.