Fuck. Glad nobody was hurt, but that really sucks. Feels like you've been raped when that happens. There really wasn't much you could of done, besides from knowing who's in your office at all times. Maybe if you had a receptionist type person they'd be able to screen people before they come in.
Since Stamper saw the guy, can't he do a composite sketch? That should be easy enough.
Maybe it's time to buy each employee a taser gun, and buy Stamper some balls.