Re: So he was "visiting" during working hours
What if it's a keyboard which does that? Whatever the device looks like, it will tell the computer it's a keyboard. So you have three options:
1. Trust any USB keyboards, including the prospect of a malicious one.
2. Do not trust any USB keyboards, using something else to connect the trusted keyboard.
3. Go through a registration process to trust only a certain kind of keyboard. Some methods include only allowing a certain set of known keyboard IDs and therefore a randomly-chosen ID probably won't work or requiring the keyboard to enter a certain set of keystrokes to be added to the trusted list.
In any case, this has nothing to do with USB. A fake PS/2 keyboard could do all of the same things and you would have exactly the same trust problem. USB having the ability to connect multiple devices doesn't cause the keyboard attack. The closest it can get is that you can make a USB device that looks like something else, but the only way to solve that comparatively minor problem is to have separate connector types for everything which still doesn't fix the larger problem and also makes hardware a lot less convenient.