I recall some system for shutting down solid fuel motors by opening pressure reliefs and, after that, maybe some cooling gas would finish the job. At the least venting the pressure alone would eliminate the thrust. Not sure if it was investigated for the manned missions. Some propellants require both heat and pressure to release energy.
The freezing problem with the waste disposal suggest to me that the very best engineers were put on other things and that, perhaps, younger engineers, who have never had a urine flow issue, worked on the toilets.