In 14000 hours on the 757/767 I never heard of anyone even thinking of deactivating the over speed warning. Why would you want to? The only way to stop the warning was to SLOW the aircraft to below the MMO/VMO. The drill was simple close the thrust levers and pull speedbrake. Interestingly the limit on UK certified aircraft was lower than FAA certified ones, Mach .84 rather than Mach .86.