If you tell them you aren't interested more then twice (My Brother-In-Law and friends work for a company that requires they be turned down twice), and they won't let up then you are fully justified in saying, "I'm sorry, I'm not interested I'm gonna have to let you go." and then hang up the phone, I do it myself. Hell, my Brother-In-Law does it and he's a telemarketer, if he hated being treated as such you figure he wouldn't treat others as such.
Really, as far as I’m concerned you are justified in doing your own, “I’m not interested please take me off you list spiel” and then hanging up immediately, possibly hanging around to say, “Thank You.” If they confirm that they are doing so, and then you hang up.
Trust me, a Telemarketer is simply gonna shrug and move on to the next number, it much more preferable then having somebody let loose with all their annoyance using four letter words at you.
If they won't let you go after you have repeatedly let it be known that you aren't interested they are being rude, not all telemarketers do this (For instance the company my BIL works for, in fact he can get in deep trouble if he doesn't let you go after 2 rebuffs), which is my point, not all telemarketers are the same and it is wrong to simply label them all as scum. (Just like it's popular to do with Lawyers based on the more visible ones, but just because it's popular doesn't make it fair or right.)