Keep them on the line... Make them think they have the sale. Get all the way to the point of having to pay. Ask a lot of stupid questions over and over. Wait until you can hear the frustration in their voice as they answer the same question you already asked 4 times. Then, when they try and close the deal, hit them with something like;

I told you pricks to take me off your list a dozen times. I don't plan on buying shit from you and every time you call I will waste your time like this.

They will not call back... works for me every time!