Like the others have said, please be careful when talking to Witnesses. I know all too well how great it is to be right and win an argument. But, arguing and being rude - or trying to "shut them down" - will only be counterproductive.
They are taught they must go door-to-door and proselytize in order to be good Witnesses. Shutting them down is tantamount to someone who is more knowledgeable than you running circles around your discussion when you are trying to kindly teach them the way to heaven. Please don't do it. Winning an argument will puff you up, but the Witnesses will still be headed for hell and may be pushed away. What's worth more to you - winning an argument, or guiding someone away from eternal separation from God? If winning an argument is more important ... I think, the goal is NOT to win arguments, enjoyable as that is. The goal is to lead them to the TRUTH - that Jesus is God in the flesh, that only He is the way to heaven, and that He desires them to be with Him forever.
I could give umpteenth arguments for talking to Witnesses, but they will do no good without love, as Paul said in 1 Corinthians. They will do no good without the Holy Spirit's guidance. I was discipling a friend this past year who has Witness relatives. Sometimes I gave him things to ask, to help them think critically about their decisions, but I told him I could not give him the perfect argument; even if I knew everything to say, they might not want to talk about that. He told me they dodged "good" arguments and spouted pat answers, so that might not work anyway.
I did have an experience where I stopped and talked to some Witnesses at a street display. I told them I was a Christian from the local convention in town (which I was). I spent a long time talking gently with them about Jesus and why He had to die. I made my point by asking first, to wit, "Jehovah is the only name by which we may be saved, right?" They said yes. Then I kept prodding: if Jehovah is the only name by which we may be saved, why did Jesus have to die? He didn't want to if He didn't have to, the Bible says. But Jehovah allowed Him to die anyway. Yet, according to Witness theology, there was no reason for Jesus to die, whatsoever. I could tell I got the gears turning, and they said the questions were good but they could not answer them without further study.
At the end, though, they said we'd had a nice discussion and thanked me for being nice to them. Wow! That meant more to them than any argument I could give, I think.
Best of luck to you when they come knocking. I hope you can lead them to Christ!