I needed to ignore an USB storage device, or at least disable the automount feature for it, because I wanted the USB device to be available for VirtualBox.
There does not seem to be an device-specific ignore functionality, but in my case it was enough to temporarily unload the USB storage kext:

sudo kextunload /System/Library/Extensions/IOUSBMassStorageClass.kext

This will disable all USB storage media temporarily!
When you are finished using the USB storage device in VirtualBox, shut it down, and then do a

sudo kextload /System/Library/Extensions/IOUSBMassStorageClass.kext

to restore the USB storage functionality.