Can I stop someone getting remote access? Is there something I can turn off so it's impossible?

Remove your computer from modem, router, WiFi, Ethernet or mobile broadband. Disconnect as fast as you can with all that you're connected to and stay offline! The remote person could have opened up ports, made fraudulent certificates giving them Admin rights, put in keyloggers, may have all of your passwords, may have made many registry modifications including remote registry entry. They may have changed your firewall and router settings, the list goes on and on. Find a trustworthy, very knowledgeable computer repair person to re-format to a DOD level shredding and re-install your operating system. It may cost around $100 to $150. Learn how to protect your PC in all areas.