Easyest is to dissable the jump key. But then u have to set 'default controls' on 'no' in the behaviour. And simulate (thats an action in sprite > platform) all the keys. Ex. : When left key pressed > simulate left.
For the jump you need a variable that you set to zero on start up. And set to 1 when player jumps.
So logic for jump is this.
In the same event: 2 conditions combined :
Up key pressed
variable = 0
set variable to 1