For some reason Rolls hasn't used the 'clashing blades' to slow the turbine in the event of a shaft separation. I always figured it had something to do with the 3 spool architecture on the RB211 and Trent, but that wouldn't explain the Spey...
The Trent series uses an electronic Turbine OverSpeed protection system on both the LP and IP shafts that instantly shuts off fuel if a shaft separation is sensed.
Of course nothing is instantaneous. It has to do with a fuel fed pressurized system and its blow-down rate through the various turbines and their inertia to speed up when their driven load is suddenly released. Match this against their ability or inability to move into stationary vanes when released from the load and then solve the problem.