Unlike most sorts of spam, 419 scams rely on being sent from a working acount. Since an actual human has to receive and reply to incoming messages for the scam to work, it is possible to get in touch with the would-be fraudsters and harass them back to humorous effect.
However, it occurs to me that there are several more drastic measures available. Instead of responding to occasional fraud attempts, we could automatically detect these fraud probes and send in a chatterbot to automatically strike up a conversation, effectively wasting all of the scammer's time and making the process a great deal more labor intensive.