I think if we reply, it opens a door of communication. Through that door will come any threats of litigation. We can therefore use it as a weathervane. And also as a door.
It is the excuse. You can put it any way you want it; don't dish it if you can't take it, don't start none won't be none, fucked around and found out. But the fact of the matter is, in this world we must be kind first, and be kind to all, but as soon as the other person foregoes kindness towards you in favor of hostilities, you destroy them. Mercilessly, utterly, as completely as you can. You let them know you are kind as a choice, not for a lack of spine. And move on.