sleep 5 is IMHO the shallow end of hackback approaches. As suggested by hossman returning a clear message to your spammer is an option, largely to disorient or bloat a bot at the other end. One of many discussions on slashdot include things like sieving through identified spam , following the 'unsubscribe' links but mangling them to remove the http://spamdomain.here.com/snoopy?victimid=spamvictim@there.com to POST large annoying messages to thwart the good-hit counting.

When all is said and done, doing anything to defend yourself against spam is joining an arms race. Be careful.