Is it even necessary to reply at all? I find it quite outrageous that random strangers could tap into my time like that. Or if a reply is necessary, could I charge for it? (I am not the original poster, just hypothetically speaking if something like that happened to me).
So long as your reply is short, balanced and factual then a reply is better than not replying. If nothing else it's extending the courtesy that we'd all want extended to us if we were making a complaint (saying, sorry, no, you've made a mistake), but also you're kidding yourself if you think these things go away when you ignore them. Reality is that it's more likely that they'll escalate.
In terms of could you charge for it, no of course not, sadly that's not how any of this works. You can throw out this sort of threat with little or no comeback.
Their lawyer is going to be looking for an excuse to bill more time on this issue, so if you don't reply, you will definitely hear more from them. So eventually you'll have to do something anyway. The sooner you respond, the sooner it's going to go away.
