The thing is, if you manage those things remotely, so can an attacker. I imagine it would not be impossible for a sufficiently skilled and determined attacker to remotely re-enable the USB ports.

If you are paranoid enough or have actual reason to believe somebody would want to invade your network, epoxy is one way you can be really certain no one can hijack your network via an infected USB flash drive.

EDIT: Of course, if gluing the USB ports shut is all you do stop attackers, I am pretty much begging for trouble. And as somebody pointed out, disconnecting the USB ports from the main board, possibly disabling the pins is probably a better idea, as well as disabling the USB controller in firmware and locking the BIOS / setup, if it is part of a ... let's say comprehensive approach to securing your network.