Honor system and hotel card swipe data audits. You can't swipe in within the shut-in time or you will get in trouble. It's actually quite easy to enforce. Hotels are more than happy to hand over the data to the company in addition to CCTV.

Violate the rules and get caught by the safety team, CAA is contacted for safety violation, you are fired or heavily discplined(and this is recorded for you...going to be hard getting a job in the future), and you now have a record that may deny you any licensed aviation position(and possibly more).

BR has actually screwed over many people's future through strict reporting of your discipline violations and your recorded incidents with the company in addition to "leaking" your irresponsibility to the media that may or may not be broadcast with your photo(your eyes are blurred of course and only your last name will be used). Media will be more than happy to pick up a safety violation story and run it to make a big deal out of it.