You could try and use the kill() method:
Also you can store your reference to the tween in a variable:
var myTween = TweenMax.to($('#score'), 0.25, { fontSize: '56px', onComplete: myFunction });
function myFunction(){
myTween.kill();
myTween = TweenMax.to($('#score'), 0.125, { fontSize: '28px' });
}
For more info checkout the GreenSock TweenMax Docs.