A dummy plug was going to be my suggestion.This gets me speculating. It might be interesting to research the actual wiring in the laptop. IF the internal microphone is electrically routed through the jack on the side, you could cut off the microphone with a dummy plug in that jack. I'm skeptical of that, though. Even if it is, there could be a feed taken off as well, for just such a purpose. You'll probably wind up wrenching it. You might be able to put a small switch in, as well.
Dummy plug: via the OS you could still flip the setting to give priority to the built in mic, ignoring the plug.
Removing driver: software can easily seek out the driver and install it.
Disabling in BIOS: the OS can interact with the BIOS. At one point Dell had a tool you could run, and on the next reboot it would update the BIOS. So one could imagine a clever hacker/gov could run a tool that would ensure the mic was enabled upon reboot.
The only way to be sure would be to remove it physically.
But the you get it on the weeds with a speaker is a lot like a microphone. So one could make the case that a really clever knave could turn a speaker into a mic.
But now we are getting super paranoid.
How about some white noise? An earbud or small speaker right at the microphone to overwhelm any sound the corporate spies are trying to pick up on. A little heavy metal to lull them to sleep.
Whether that light will function accurately when the powers that be want to monitor is still an open question.
If you are really concerned, I'd either be far away from the laptop when having confidential conversations, or have it powered down (sleep or hibrnate would be fine).