Your response can take a number of forms. The first and most important thing you should do is to take a break and cool off. Replies written in the heat of anger are seldom any better than the postings which inspire them.

Now that you've cooled off, go back and consider the offending message again. If it's nothing but baseless arguments, then the best response is to completely ignore it

If, on the other hand, the posting contains inaccuracies which you feel need to be addressed, then it is perfectly reasonable to send a follow-up message which does so.

However, the ideal approach is to ignore any hysteria, and stick with the facts. Be reasonable and rational, point out the errors, and avoid any personal attacks.