So don't answer. No-one has thumbscrews on you to make you reply.
If you don't like the question, and you don't want to be drawn into more on the subject, don't even look at that thread again, then you won't know what the question was.
It may surprise you to hear this, but no-one will even notice that you have not replied. The conversation will move on. Other people do it all the time, and they hold back from commenting when someone says something stupid, too.
There is no law that says you will be thrown in jail for not answering a question you don't like - or even a question you can't be bothered answering.You don't have to go and find out the right answer - it is not an exam. All replies are voluntary. You don't even have to say "I am not answering that" Just go and do something else.
There. You now have permission to say nowt! does that make you feel better?