Not that I make a habit of talking to them at the carts, but I found this method seems to get around their apostate radar...
I ask them "So who's Jehovah? Is he, like, your prophet or something?". This makes them think I'm clueless about religion, because any theologian would know the word Jehovah.
Then I tell them that my family are into Scientology and dont talk to me because I'm considered a "suppressive person" and "disconnected" from the religion. You'd be surprised to learn that when I said this in the past the JWs said "that's horrible". I almost broke character.
The only time a conversation got shut down was when the JW swore blind to me that the bible doesn't advocate slavery. Of course, we all know it does. When I pointed to the scriptures he explained it away as "they were treated well, more like servants" (tell that to Hagar).