e.preventDefault(), and return false; in the function
$(document).on('keydown', function(e){
if (e.which == 13){
e.preventDefault();
return false;
}
});
or if you mean just the event binding
$(document).off('keydown');
if you mean just for that particular binding you can set a namespace
$(document).on('keydown.mykeydown'...
$(document).off('keydown.mykeydown');