If there is a lock/stop, it should be referenced in the manual.
Many years ago I had a Konica-Minolta Scan Multi that I bought used from someone who didn't know that there was such a mechanism. They shipped it to me unlocked, and it required several hundreds of dollars to repair.
In that case, the lock could only be enabled through the Konica-Minolta software for the scanner. If that is the case for the Coolscan V, you will need to be able to run the original Nikon software in order to engage the lock.