Up until 9/11 they did not even track who left.
Now they do.
It is very easy to just coordinate the computer database of the BCIS to generate a letter to the employer upon the "terminate date". The terminate date will give the "guest" 30 days to leave the country (ala the current bag and baggage order of the immigration courts). If a departure is not recorded the employer is sent a letter which requires them to swear UNDER PENALTY OF PERJURY PERSONALLY AND CORPORATE PENALTY the employee has left.
IF (very big IF) they think this out is can be REASONABLE. Those that remain automatically have a felony warrent put out for them and an immiration hold.