Currently any domain, or subdomain with :8443 affixed directs to the Plesk control panel, is it possible to turn this off except one "control domain" ?
There isn't a way to restrict this to just one domain. If you are looking to restrict access and improve security of the Panel:
1. Use "Restrict Administrative Access" under Security settings to limit which networks are able to login.
2. Enable the Google Authenticator extension to add two factor authentication when logging into the panel.
3. Enable the Security Advisor extension to generate a lets encrypt ssl certificate for the panel.
4. Use a good Firewall to block off the port completely except for networks that you allow.