How do I restrict a company level user from accessing specific organisation units?

This feature is available on Premier and Master editions
The User Org. Unit Permissions screen allows you to specify which employees the user will have access to on Organisation Unit and/or Region level.
Navigate > Config > Security > Security > User Org. Unit Permissions
Step 1: Ensure that the "allow access to all employees" toggle is switched off.
Step 2: Click on the "allocate access" icon for the company level user.
Step 3: Select the checkboxes for the Organisation Units/Regions that the user should have access to.