After you shoot a Bludger at someone, press [Away] and it will hit the person almost every time.

Hold [Down] and it will always hit.

While you are hitting the Bludger, let it drift out in front of the person, then pull the Bludger towards you. It will hit almost every time, and if it does not, it will go in-between the beaters frequently, and may give you another chance to go to the side and hit them.