Chances are, if that rat didn't do any damage beyond what you listed, the rat wasn't really "biting," but just trying to see what the snake was - they tend to explore their world with their mouths.

In any event, check you husbandry. Ball pythons shouldn't refuse food like that unless something isn't right on the keeper's side of things. Make certain your temps are where they should be (measured with an ACCURATE thermometer - those crappy stick-on thermometers don't count), and that the snake has plenty of places to hide on both the warm and cool side of the tank.

Hope that helps.