I get a lot of telemarketing calls from India. I figure that a call with me telling them to go away probably only costs them 10c.
what I do is say "can you hold on a moment" and I put the phone down and go watch TV or play on the computer. after a while you can hear the disconnect tones from the phone and I hang up. doing this I figure the call costs are a lot more for them.
Also, all the marketing mail that I get with a pre-paid envelope in it. that only costs them postage if it's used. so I do. I send them blank forms, empty envelopes or junk mail from someone else (like a pizza shop or one of their competitors.