I ran a telemarketing sales floor when I was 22 or so (and almost lost my soul in the process) so here's how I deal with them now....
The most effective way to stop telemarketing is to request that they put your name/number on their
do not call list. Legally, in the states anyway, they have to. Any further calls from the same company would allow you to file a formal complaint with the Attorney General and put said company at risk of getting some pretty serious fines.
Being polite actually yields much better results than trying to annoy them back, as well. These people have heard "**** off and **** you" and been screwed with all day long, so they actually take notice when someone politely declines. They may even try to ensure you don't ever get called back.
With that being said, if you really want to ruin their day, keep them on the line as long as you can and pretend you're interested. Change your mind right before you commit to what they're selling. In the 20 minutes they spent trying to convince you, they could have had 3 other people signed up.
One last thought. Most of the call centers are completely dependent on automatic dialing machines. If a telemarketer calls you and hears your answering machine, they hang up and mark the call as "answering machine." The system will usually call you back again later in the day, and will continue to do so until your phone number is (by the caller) marked as something different. Marking a call as "disconnected number," for example, will tell the auto-dialer not to try that number again. Annoy the crap out of a telemarketer and rest assured, you're getting marked as "answering machine" and someone else in the building will call you right back.
This is one of the best I've heard, I think it was actually posted here some time ago:
http://www.metacafe.com/watch/ ... be_pranks_a_telemarketer/