Re: I can save them $4,500 per month
Another method: Just have a timed announcement at closing to the effect that: "The office is closing in half an hour. Anyone still on premises after that will be escorted out by security."
Sounds too manual to me. A timed firewall rule for the office network will do the job just as effectively, unless your employees actually don't have a home to go to, which is another discussion for another time. (Yes, Silicon Valley, I'm looking at you.)