Papaya and pineapple does nothing to prevent to treat hairballs. The enzymes simply can't break down or dissolve fur, at least not at temperatures that occur in the living rabbit. Some have added sugar which can be bad for rabbits as it upsets the bacteria in the intestines.
The best things to prevent hair balls are lots of hay, water, and brushing. The hay keeps things moving and pushes out the fur. Water keeps them hydrated and keeps the gut moving. Brushing helps prevent them from ingesting too much fur in the first place.