What I do when I get an annoying telemarketer who won't take "No" for an answer is I will tell them to hold on for whoever they ask for and just leave the phone sitting on the table, off the hook, and never go back to it for a while. They will finally hang up. If you do that enough, they will just stop calling. It works for me every time. Either that or I will just keep saying, "Hello! Hello! Hello! Is anybody there? Hello!" and finally they will hang up.