On some linux distributions [0] microcode is loaded early during kernel boot. So it's a matter of pinning the package to a previous version. No idea what happens with BIOS/UEFI updates though, although you can roll these back too. Obviously the "proper" way here is to have open source microcode but I can't see that happening anytime soon.
I know this way of disabling mitigations on Linux (nevertheless thank you, I well might be unaware of this feature, I only learnt it some years ago).
However, I doubt you can disable microcode-level mitigations this way. I believe it only affects the kernel level.