Diagnosis
This is a software setting, not a fault. Cruise control activates automatically after holding a steady speed.
How to Fix It
You cannot permanently 'disable' it in the stock firmware. To cancel it during a ride, either brake lightly or tap the throttle. To prevent activation, vary your speed slightly every 10-15 seconds.
If That Didn't Work
If it activates at wildly varying speeds, the throttle or speed sensor is faulty. See the sticky throttle or error code fixes.
Heads Up
Unexpected cruise control is dangerous in traffic. Practice canceling it until it's second nature. Always be ready to brake.