If Jehovah's Witnesses come calling, ask them in and go over a few things with them.
They tend to use their own 'New World Translation' but you can use any version of the Bible, I've taken these quotes out of their own Bible to help avoid arguments about 'translation'.
(1) Ask them if they believe they speak in the name of Jehovah, they should say they do
(2) Read and discuss Matthew 24:3-5,11,24:
3 While he was sitting upon the Mount of Olives, the disciples approached him privately, saying: “Tell us, When will these things be, and what will be the sign of your presence and of the conclusion of the system of things?” 4 And in answer Jesus said to them: “Look out that nobody misleads YOU; 5 for many will come on the basis of my name, saying, ‘I am the Christ,’ and will mislead many.
11 And many false prophets will arise and mislead many;
24 For false Christs and false prophets will arise and will give great signs and wonders so as to mislead, if possible, even the chosen ones.
Confirm that this is saying that misleading people with false information makes one a false prophet.
This is a repeated theme in the Bible, as in Jeremiah 23:25-29:
25 “I have heard what the prophets who are prophesying falsehood in my own name have said, saying, ‘I have had a dream! I have had a dream!’ 26 How long will it exist in the heart of the prophets who are prophesying the falsehood and who are prophets of the trickiness of their own heart? 27 They are thinking of making my people forget my name by means of their dreams that they keep relating each one to the other, just as their fathers forgot my name by means of Ba′al. 28 The prophet with whom there is a dream, let him relate the dream; but the one with whom my own word is, let him speak forth my word truthfully.”
(3) hopefully they have agreed with you about false prophets, how to identify them, and that they claim to speak in the name of Jehovah. If so, then ask them about their religions repeated failed prophesy (see details at linked page) about the end times.
I'm sure they will hem and haw about it... move on
(4) and ask them about Deuteronomy 18:20-22
20 “‘However, the prophet who presumes to speak in my name a word that I have not commanded him to speak or who speaks in the name of other gods, that prophet must die. 21 And in case you should say in your heart: “How shall we know the word that Jehovah has not spoken?” 22 when the prophet speaks in the name of Jehovah and the word does not occur or come true, that is the word that Jehovah did not speak.
Have the Jehovah's Witnesses not spoken in the name of Jehovah?
Have their predictions not failed to come true?
Are they not then false prophets?
But because you don't believe that nonsense, you refuse to demand their death for simply being wrong because that would be evil!
(5) If you aren't bored yet ask them about 1 Kings 18 and ask if they can provide you with such a demonstration that THEIR claimed god is real.
If they protest that you can't "test god" quote E‧li′jah from section  and say: “Call at the top of YOUR voice, for he is a god; for he must be concerned with a matter, and he has excrement and has to go to the privy. Or maybe he is asleep and ought to wake up!”
Point out that excuses weren't accepted on behalf of other gods and the consequence was (according to the Bible) the mass murder of hundreds of priests.
1 Kings 18:40 Then E‧li′jah said to them: “Seize the prophets of Ba′al! Do not let a single one of them escape!” At once they seized them, and E‧li′jah then brought them down to the torrent valley of Ki′shon and slaughtered them there.
Ask them if that seems a fair punishment for people who claim speak for a god that refuses to set bull meat on fire when prayed to?
Demand an answer to that and they will have to say that it was God's will and so it was right and just, and then point out what hypocrisy it is to refuse to be held to the same standard you're religion claims it has held others to.