Hell, if you really want to get cheeky and piss off the lawyers, ask them:
Please at your nearest convenience reply with all the evidence you have so far accumulated thus far that indicates the person with which you have contacted has indeed been the one to violate your client's IP rights. Please cease all further communications until this request has been fulfilled.
I get these from comcrap weekly.
They are toothless. IPs aren't people.
Hell, if you really want to get cheeky and piss off the lawyers, ask them: