Communication and relationship between client and Exchange is regulated by User agreement, which every user accepts at registration. There is no way to register without accepting the user agreement. However, rarely, some clients ignore and flagrantly breach the User agreement.
The client is clearly provoking a conflict, ignoring all suggested options of settlement. Earlier this client said that all information, violating the user agreement had been removed, but the investigation found out that, on the contrary, there is much more such information now. All further discussions of this issue will be held solely with the client and only after eliminating all negative effects.
FYI, it's customary to pay the customer his money before expecting negative public complaints to be removed. He's using this as leverage to get what is rightfully his. I see nothing wrong with that.
Do you seriously think you can hide behind your terms as a pretense to rob your customers in open sight? Merely stating that you will steal customer funds for breach of terms -- terms which you can and have changed suddenly and without notice -- is not legal justification to do so. The actual damages arising from that alleged breach are not equal to whatever you decide to steal from your customers. I don't know who your legal counsel is, but your actions would not be legally justifiable in any respectable court of law.
I recommend you tread carefully here.