You can turn off your clients' ability to send you direct chat messages by going to Settings > Telehealth & Messaging:
Under the Direct Chat Access section, uncheck any clients who should not have access to send you direct messages you from their Client Portal:
Clients will still be able to send and receive messages in group chats.
Disabling Direct Chat for New Clients
Optionally, you can disable direct chat for new clients by checking the "Disable direct chat for new clients by default" option in the Chat Messaging section: