On most Boeings simulators I have 'flown' there is a big red guarded switch on the back of the centre console that stops the SIM in its tracks.
As a young FO I was PNF when the trainer started piling on various faults for the seasoned captain in the LHS, it soon became evident that the exercise was bordering on improbable if not impossible whereupon the captain in the LHS leaned back in his seat, reached behind the console and switched everything off! There was a brief exchange between the training captain and the captain in the LHS and the session continued in a much more realistic fashion.