This was a conversation posted to Project Spark forum and recovered by The Wayback Machine.
This was discussed on the forum and two solutions were put forward. The conversation got so argumentative and unpleasant I am not going to post it here. Lucid Stew’s solution
WHEN [left stick] DO [in direction][left stick]
seemed to work, but only if you do not move the camera from its default facing north position with the right stick. If you move the camera around so you are in front of the player then try it the controls appear to revert back to normal again. So I do not recommend using this method unless you have a locked off camera angle (the right stick cannot move the camera).
Mindcraft Max posted 2 different ways to kode inverted controls which did work which are:
WHEN [negative][left stick] DO [move]
WHEN [left stick] DO [move][at speed][negative]