Open a Apple Support ticket.
You should be able to disable the DGPU via BIOS Settings. If there are no BIOS Settings to disable the DGPU then you don't have the option to disable the AMD Discrete GPU Card.
Which AMD Discrete GPU card that you have installed that you say it is defective?
Does the AMD GPU card run normally without being in a Windows Boot-camp environment?
Have you tried running your Laptop's Diagnostic software to see if it shows any hardware or software issues?