Sometimes I get creative. Someone selling siding, I live in a brick home. Someone peddling phone plans, I do not have a phone (this one is really fun!) Sometimes I use a variation of this one, too:
Dip: Hello, I'm selling long-distance services.
Me: I don't have a phone.
Dip: You're talking on the phone right now.
Me: This isn't my house; I don't even know these people. I'm just here taking stuff, and I'm kinda busy and in a hurry so I don't have time to talk right now.
Dip: < gurgle >. Eventual < click >
I told a telemarketer once I was currently in an armed standoff with the state police and asked if his company did any direct marketing for a good legal agency... I was in great need of good legal advice.
Other times when I am in no mood for long calls, and I suspect the caller is a telemarketer, I just answer the phone, "Hello. This is Pizza Hut! Take out or Delivery?" That usually stops the call dead ASAP