When you go to play a round, don't play a RANKED round. The option is available to play either a ranked round (counts in average) or a PRACTICE round (doesn't count in average.)
Hint: equip yourself with starter balls when you play a ranked round and are intending to take multiple mulligans to keep trying a shot. Mulligans start to count as two strokes on the durability of your golf balls, so if you are using 600 credit balls, you are going to be losing a lot of money. You won't get the spin you may be used to, but your wallet will stay full.