I want to restrict workspaces to a company manager? / Can I demote a company manager to a manager?

You cannot restrict workspaces to a company manager.  A company manager has access to all workspaces including the default workspace.  To restrict workspaces to a company manager you will need to go to the members tab in company accounts and change the permission role from company manager to manager.  

Once you demote someone from company manager to manager they will lose all access.  You will need to go into each workspace and add them to the workspace you want them to have access to.