I think the best way to cast doubt is using the book of Revelation.
Start off asking about Revelation 22:13, which states
I am the Al´pha and the O·me´ga, the beginning and the end. To anyone thirsting I will give from the fountain of the water of life free.
Then tell them you are not sure who Alpha and Omega is referring to. They will tell you it has to refer to God.
So then have them read Revelation 1:8 which says
“I am the Al´pha and the O·me´ga,” says Jehovah God, “the One who is and who was and who is coming, the Almighty.”
Then ask them if Alpha and Omega can possibly refer to anybody else, to which they will be more than happy to tell you its a title reserved for God himself. If you feel the need to further emphaize the point have them read revelation 21:6, and they will tell you without a doubt only God is called Alpha and Omega, to which you can happily agree, and they will be glad how easy your questions are.
Then ask them to read Revelation 22:12-15, but make sure you stop them at 15. It says
“‘Look! I am coming quickly, and the reward I give is with me, to render to each one as his work is. 13 I am the Al´pha and the O·me´ga, the first and the last, the beginning and the end. 14 Happy are those who wash their robes, that the authority [to go] to the trees of life may be theirs and that they may gain entrance into the city by its gates. 15 Outside are the dogs and those who practice spiritism and the fornicators and the murderers and the idolaters and everyone liking and carrying on a lie.’
STOP THEM THERE! They will will say, surely God is talking, it cant be anybody else.
Have them read on:
16 “‘I, Jesus, sent my angel to bear witness to YOU people of these things for the congregations. I am the root and the offspring of David, and the bright morning star.’”
Clearly Jesus just defined himself as the Alpha and Omega. But how can that be, they just spent the last 10 minutes explaining very clearly in no uncertain terms that ONLY God is the alpha and Omega, to which your response could be that Jesus is in fact God.
Another defense they may have is the author of that verse changed, but there is no indication of that. The only thing I can get by reading that passage is that Jesus is declaring himself to be God.
I don't think its a perfect method, but its simple as you only have to memorize the location of four verses, you can even write them down and put them in a drawer someplace if they are coming once a week. You don't need to be an expert in apologetics, and you can even let them to all the reading. If you really shake their faith or even let them know that you know what you are talking about, they will stop coming, and it is then up to the holy spirit to convict them. We never have any Jehovah's wittiness come by our house, but if we did I would apologize and tell them I had to leave for work or something and make an appointment for them to come, and when they did I would be ready.
Sorry for the long post, hope that makes sense.
PS: all quotes are from the New World Translation, I don't agree with that bible, but they believe yours has errors in it, and theirs doesn't, so let them read from their bible, since they wont believe what your bible says anyway.
Hope this helps, I think if nothing else its better than slamng a door in their face.
