If you are ever in a situation where a Witness is seemingly interested in what you have to say and believe ask them this question:
"If what you believed about the Witnesses and the Watchtower Organzation was false and there was no hope of Paradise or anything else, would you want to find that out? Or would you like to be kept in blissful ignorance and die believing it was all true."
If their answer to the question is blissful ignorance then it would be best to not get into any spiritual or doctrinal debate, since they are obviously married to the idea and will never actually budge.
If they answer that they would like to know the truth if it were all a lie, then you have something to work with.
Just a thought I had.
-Sab