If you use the stoneball on a regular Guard, then hit him with any weapon like Hammer or Mace, he will shatter and drop his Crossbow.
You can then keep the crossbow in your inventory and use it to shoot enemies.
It will only stun guards, and has no effect on an Armored Guard or some larger enemies, but will kill most smaller enemies, and is especially useful against Henchmen who are throwing things or blocking exits since you can kill them from a distance, and don't need to fight them with a shovel or something like that.