Geez, just hang up! You don't have to engage a telemarketer. It doesn't have to be rude - I just say "Not interested, thank you" and hit the disconnect button, don't even wait for a response.

It actually benefits both of you to hang up, if you have no intention of donating/buying/whatever. If they're on commission or quota, they can move on to the next call - if not, then no loss.