The easiest way to stop Jehovah's Witnesses from calling on you - At least in the UK - is to politely tell them that an 'apostate' is present at the address.
To the JWs, an 'apostate' is an EX Jehovah's Witness - Someone who has left the faith, and their own rules forbid them from having any contact with apostates.
As such, they will make a note of your address and it will be removed from their maps as a 'Do Not Call Upon' incase the apostate opens the door next time.
Its a simple, elegant and polite solution, which uses their own rules against them, and ensures you your peace and quiet without having to be rude or disrespectful yourself. Once they are told that an apostate is present, they will NEVER knock again
