Answer the door naked and carrying weaponry. (immediate results)
Pretend to be the slowest talking person in the world and see how long their spirit of Christian charity lasts.
Answer every one of their questions with "What do you mean by that?" This might take a while, but you and your loved ones can have fun placing bets on how long it takes for them to get flustered and leave.
Ask them to explain the story of Elisha and the Forty two children. You may have to resort to another method to actually get rid of them, but this will definitely make them sweat. (2 Kings in Chapter 2, umm... somewhere near the end).
Excuse yourself from the front door and DO NOT come back.
Make a series of increasingly reprehensible fake phone calls. (bookie, order for pornography, drug deal, obscene call, and if they are STILL there, a tearful confession to the police for the murder of the last Witnesses who visited you.)
Pick an often repeated word in their vocabulary (God, Jesus, heaven, it, the etc.) and giggle whenever they utter it. If they ask you what's going on, say "nothing, why?" in very even tones, and giggle again.
Same as above, except say "beep" instead of giggling.
Guys can show an intense interest in their spiel. Part way through, begin putting on make-up, hosiery, a dress, the whole works. (make encouraging noises [uh huh, I see...] throughout and if they ask you what you're doing, pull a #7) If they're still there when you are done, Ask them if they would please kindly leave as you have a hot date in ten minutes.
Look smug and tell them that your God can beat up their God.