Block USB storage?

Corrupt the usbstor.sys driver and corrupt the system file recovery. (it will work, but you really shouldn't do that lol)

If these machines are being used by untrusted users as you indicate, since you must block USB storage devices, you should not allow them administrative access to the systems. You should use a combination of group policy and user account/group access restrictions.

You should also lock the bios with a password, and disable booting the laptop in any way other than the installed OS on the hard drive.

If the USB log utility can be run via the network for a remote computer, then you would check the log without anyone knowing. You can just connect to your computer via TeamViewer and execute the utility to see USB activity on the others. But I'm afraid it is a local only utility. Any similar utility that can be used for remote computers?

The best is to block access to external USB storage. If anyone attempts to connect a pen drive will fail. Later in the day you can run the USB log utility to see if there was such an attempt and by the time was done you could find out who did it. That's why I told you to combine the two utilities. It makes little difference if you discover it the moment it is done or later, as long as he cannot steal data. You can punish the guilty later. You could also limit the maximum size of attachments in e-mail or forbid them altogether, so they cannot steal data by e-mail either.