How do you UNDO a conditioned response?

He's 11 & ran barrels when he was young. I ride a racetrack. I prefer to lope, but he wants to gallop, so I let him. He'll go just so far, take a sharp left (unexpected) turn, circle & return to the racetrack without ever slowing up. A little farther & he'll repeat. A little farther & he'll repeat. I'm on a damn Merry-go-round! He's running barrels in spite of the fact that there ARE NO BARRELS. He neck reins very well except here, with this. I've neck reined him with all of my strength & have spurred him HARD on the left to keep him from turning. Nothing works! My cues are being completely ignored & I'm left feeling like I have no control over him. What else can I try? It's obviously a VERY STRONG conditioned response. Blinkers? A claw? Help! Dizzy in Louisiana.