Stop an executing function

Stop an executing function

There's a piece of code that runs as a function called upon key stroke, I want to stop execution of the function on pressing another key stroke both outside and inside of the running function, here's the pseudocode. 

$(document).on("keydown", function(e){
switch e.key {
case "a" : /*run*/ myFunc();
case "b" : /*stop*/ myFunc();
}
})

function myFunc(){

Some Loop(){
$(document).keydown(/*stop*/ myFunc());
}
}

How do I make it stop in both ways, obviously to run doesn't need anything, but calling the function, but how can I stop the executing function ?