I found a nice article on Lifehacker today that explains how telemarketing works and provides the best way to get rid of them. I really dislike getting phone calls trying to sell me something I do not need or want. They waste my time and deprive me of much needed down time after a long day at work. I agree with virtually everything in the article except the part about not showing them any human kindness. These people are getting yelled at, hung up on and otherwise ignored all day long.
This situation reminds me of a quote I heard saying that basically I might be the only Bible somebody gets to read. If I'm ignoring or hanging up on one of these people, I'm not exactly being true to my own values. What I recommend is politely ending the call. You treat the telemarketer with the utmost respect, even asking them something like how is their day going and then firmly state your need to hang up and make sure they understand you do not need them to call back later. The risk is that they put notes in your file that might lead to more calls but if you have the energy to spare, spending a few extra seconds being hospitable before you terminate the call might make all the difference to somebody suffering in one of those telemarketing "boiler rooms" where they make hundreds of calls a day.