I think you have your answer. Simply remove the source by disconnecting the inflator (same for drysuit or pop a seal). Also you have a few other ways to expel air from a BCD, practise them all gloves on and gloves off and learn to inspect and maintain your bcd. In the time you pull your knife you could of disconnected the inflator. So now if you cut your BCD what are you going to do for surface floatation other then drop you weights, tank, reg and other equipment Good thing that wetsuit floats but man thats one expensive dive. But then your life ain't worth all that equipment anyway