Be a broken record, keep saying. "No, I can't answer any questions, I do not have freedom of speech"
If they act really dense and want to know why, explain very briefly, and very calmly, why the JWs don't have freedom of speech, refuse to go into much detail other then "I learned my lesson last time I answered a JWs question"
I did this for a while with good results, it shut my JW's right up. I also believe it got them to think, just a little bit