when telemarketers call, if you have the time, try to keep them on the line as long as possable. they make their money playing the percentages (x sales for every y calls), most people will hang up right away if they aren't buying, so marketers can get 4 or 5 calls every minute.
if you keep them on the line for 1/2 an hour you probably cost them 150 calls (10 minutes is my personal best, but the wife is awsome, she kept MCI on the line for almost 45 minutes)
it's bussiness and bussiness is all about profit, when they realize it's unprofitable to call you the calls will stop.
BTw- on a simular note. junk mail. my wife takes those postage paid return envelopes, fills it with everything included in the mailing(even the original envelope) writes on it that she doesn't feel it's her responsability to throw away their junk, so she's returning it. and sends it back.