It is a GlovePIE script, so you download the program GlovePIE, put this script into it and run it, then you launch the game. GlovePIE just runs in the background to capture the inputs and run the relevant parts of the script when those inputs are detected.

Tried it, it sorta works, I don't know if it's my mouse's polling rate or DPI being too high but it doesn't completely deactiveate the Y axis, I alt-tabbed and I can see the cursor like flickering down from the imaginary horizontal line. It's definitely better than the way it is by default so thanks anyway.