The pilot stated that the approach and landing were uneventful through downwind, base, final approach, and touchdown. He landed on the center of the runway, but the airplane started to drift/pull to the left. He pressed right rudder with some brake pressure, which slowed the drift to the left, but did not totally stop it. He continued these inputs, but the left main tire caught a snow bank. After traveling a few hundred feet, the airplane spun 90 degrees into the snowbank. The operator reported that there were left tire skid marks as the airplane exited the runway, and opined that either the pilot or passenger had their foot on the left brake pedal. The operator issued a new caution to all of its instructors and pilots regarding the use of brakes on touchdown and roll out, and reminded them to brief all front seat passengers to keep their feet off the rudder and brake pedals.