On your router, configure it so that it gives Computer C a fixed IP address. Some routers call this "IP Reservations" or similar. Then configure the firewall on computer A to reject all connections from Computer C's address. You may be able to accomplish this on your router, depending on the capabilities of its firmware, but you can definitely accomplish it by configuring firewall settings on Computer A.
You could also make sure Computer A has strong passwords on all accounts and isn't running vulnerable software or software allowing anyone to access files on the system.
If computer C is likely to change its MAC address in an adversarial manner, then one option is to have a separate wired and wireless router, and configure its DHCP server to hand out a different range of addresses from your wired router. You can then tell Computer A's firewall to reject any connection from any IP in the wireless range.