In the Warehouse, you would block the area with a chain or tape and a sign so that employees know that counting is in progress.

In Odoo, you can mimic this process by renaming the location, then rename it back when you are finished counting.

While this does not block Users, it does alert them.  

Since the chain or tape in the Warehouse does not stop employees from taking something while counting is in progress, you are both matching the real-world process and leveraging out of the box functionality (ie: free to implement). 

They may have to take something and tell the employee responsible for counting, especially if it is crucial for the business and more important than potentially messing up the count.