Be firm, but cordial. Tell them you are not interested and the repeated phone calls are a waste of your time as well as theirs. If it keeps up tell them you feel like it's getting to the point of harrassment. Tell them you will call the better business bureau. Look into blocking the calls. The important thing is to not give them the impression you are even the least bit interested in doing business with them. Ever. There is an opportunity cost involved with them making the calls. Sooner or later they are going to realize it makes no business sense to keep this up.

I guess if that still doesn't work you could just tell them to get fucked. No reasonable person is going to keep calling to be verbally abused.