Just checked this on the Z620 I'm sitting at..... I believe the navigation will be the same for you.
Part of the security layers with HP workstations is that you can turn a bunch of things off so that they appear invisible on a hardware and software level. At least on the Z620 here is what I'd do: F10 into BIOS/ Security tab/ Device Security/ USB3 Controller/ change to Device Hidden/ F10 to save in that subsection/ Esc back to File tab/ Save Changes and Exit/ F10 to really save the change.
Now reboot.... with some other projects I've used this method makes the "device" no longer detected by the OS or even by the computer itself (it is no longer listed in Device Manager). For these type of changes I tape a post it note to the inside of the side cover so that at some time in the future I'll remember what I did.
I'm sure you know that there is a BIOS setting for whether the workstation restarts after power loss/return or stays shutdown.....