Option 1: First of all you need The Sandman.
https://wiki.teamfortress.com/wiki/SandmanAfter wards, you need to get rid of your own ball by clicking Mouse 2, then you need to pick up an enemy scout's own baseball to refill your meter, then stun that same Scout with the ball.
This is probably best done boosting as it would be very rare to find yourself in such a situation.
Option 2: Play as Pyro and simply airblast the ball back to the scout. All flamethrowers are able to airblast, except the Phlogistinator.