The private pilot and the CFI were practicing touch-and-go landings. The CFI stated that the student had been using too much rudder in correcting previous landings. As they touched down the airplane began to veer to the left. The CFI told the student to straighten it out and the student added too much right rudder to correct the problem. The CFI said he added left rudder and brake to try and correct the problem, but it was too late and the airplane had already begun the inadvertent ground loop/swerve. The airplane departed the runway and subsequently struck a runway sign.