I read an article, I believe in the Reader's Digest, written by telemarketer's
and one of the things they said is never engage them in any conversation. They said the best way to have them take you off the list is to say "hello" and nothing more. Let them continue to say "hello", "HELLO", until they finally hang up. I've tried it several time and find I get a kind of savage amusement from hearing them trying to figure out why nobody is answering them.