Location: NSW, Australia
Turpentine maybe? I have no direct experience with contact balls but turps was what I used to get the ink (from the instructions) stains off my HOP led poi.
It depends on what caused the stains - turps is good for oil soluble type stuff, soap and water for water soluble stuff.
Or bleach, but that might damage the ball itself - try that last and only if the ball is unusable as is.