My favorite way of handling JWs is to invite them to a discussion of their beliefs. Suck them in with interest then hammer them with questions that attack their foundations. This will stammer them. JW logic is easily shattered; a simple question or two and it unravels before their eyes. This should work if you want to stop the harassment, as JWs aren't taught to think for themselves.
Your dad will then fall back on his Watchtower pubs (which is where the grade-school logic originated) and that is when you can perform the coup de grace. JW literature is filled with double-speak, twisted reasoning, strawman argumentations, and more. But the amazing thing about what they print is that many times it is true. You will read in the WT where it is a conscience matter to vote, how divided households should raise their children in both religions, why religion should never come between family members, that all of us should investigate our beliefs and then follow our own conscience as to our path, etc.
Your dad will not be familiar with these thoughts because we he reads them it is through WT-tinted glasses. But you should still force him to read it. After all, it is from his own religious journals.
If he survives this battering of his faith, your relationship with him may have hope. Mostly, this tactic will cause the injured JW to flee from your presence, never to be heard from again. But, still, it could work...
