You could use the `hidutil` command to remap <kbd>Fn-9</kbd> to a null value

```
hidutil property --set '{"UserKeyMapping": \
[{\"HIDKeyboardModifierMappingSrc":0x700000067, \
"HIDKeyboardModifierMappingDst":0x700000000}]}}

```

Now, the source (hex) value of <kbd>F-12</kbd> is 0x700000067 and the destination value is 0x700000000.  If 

This is based on [this answer][1] I previously wrote. 




  [1]: https://apple.stackexchange.com/a/455274/119271