The means comprises a pair of spaced-apart walls which have arcuate recesses therein for receiving the flatted ends of an earth cutter shaft. Retainers fit across the flats formed in the ends of the shaft and are secured within the recesses and against camming surfaces of the walls. In response to any unwanted undesirable rotation of the shaft, the retainers rotate relative to an axial center of the shaft to force ends thereof, which abut the camming surfaces more securely into the recesses so that the retainer is jammed or wedged in place. This results in positively inhibiting further rotation of the shaft.