When someone shoots a Bludger, turn around so your Beaters are in front of you. The CPU will run the Bludger directly towards your Beaters and they will just knock it away. This works almost all of the time. If it does not, you probably turned around too late or your Beaters did not get in front of you in time. This will even works on the Firebolt level.

Hold [Down] + [Left] or [Right] and you will have a 90% chance of blocking a Bludger.