You can deactivate the sticky keys by clicking on the link in the poping window, and then uncheck "Activate sticky keys when maj is pressed five times" (an option like that).
But your users will have to do the same.
Add an object keyboard.
Make an event "on key pressed" select another key than "shift"
Action: spriteplayer => Platformer => Simulate control => Jump
This way you are handling the jump by yourself.
Note: this means the player will be able to jump through your chosen key and shift.
If you don't want that, you'll have to turn "Default control" in the platformer properties to no, but you'll also have to create events to simulate going right and left. (you can assign those events to the right and left arrows).