CHANGE THE BALL COCK WASHER.this is failing to seat or perished, this is not stopping the incoming water. this washer is located inside the shuttle assembly that is connected to the ballcock arm,The water supply needs to be turned off, cotter pin removed, end cap unscrewed off to allow the shuttle to come out. replace the faulty washer (you can buy a plastic shuttle assembly if need be) and reassemble the unit back together and test it shuts OFF BEFORE the overflow height .
wirecutterman
September 2009