Do you mean callers on your door step, or on your telephone? If it's on your 'phone then this lot will help you http://www.tpsonline.org.uk/tps/
Alternatively, say "oh yes, I've always wanted one of those, i'm really intrested - I'll just go and get my credit card..." then put the receiver next to the phone and don't come back.
One of those callers offered me a free mobile phone once. I got all excited and asked if I could have 50. It put him off his stride somewhat.
