SPAM - Stopping Rumpelstiltskin Attacks?

WaldoJ asks: "I often see spammers connecting to my mail servers and attempting to send mail to a series of common first names, usually a dozen or two at a time. A few get through, but most don't, and it's up to me to skim through my logs and manually block their IPs, since they'll inevitably return later on if I don't. Has anybody written a program to halt Rumpelstiltskin attacks after X failed addresses? Or, better yet, one that also automatically adds their IP to Sendmail's access database to block them from returning?"