I usually send a canned few paragraph response, saying how sorry I feel for them, how much I'm willing to help out someone in need, etc. Then, toward the end, once I've eaten up a few minutes of their time building up their hope that they've found a victim, I include a line like, "Unfortunately, I have a personal policy against dealing with Nigerian scammers like yourself."

Now, imagine you're a scammer who has sent out ten million of these things and gone to bed. You wake up in the morning and eagerly check your inbox, only to find that you have 9,132,965 new messages. At a minute each, that's over 52 years of forty-hour weeks of mail to go through. Since it only takes me a few seconds to copy and paste my response, and I only get a few of these things each week, it seems like they're getting the short end of the stick. If people responded to these things like this, it would be easier for the scammers to just give up and get a real job. Fight fire with fire; fight spam with spam.