In case you don’t need the knowledge base on the customers are there is an option to disable the knowledge base feature.
To disable you need to navigate to Setup->Settings->Customers and set Use Knowledge Base to No.
If you give the staff members permissions for knowledge base View (Global) then they will be able to see the internal (for staff) knowledge base articles in the admin area.
In this case, to prevent specific staff members to read internal knowledge base articles, you should not give permissions for knowledge base View (Global).
Keep in mind that if the knowledge base is enabled in the customers area, every visitor on the customers are will be able to read the articles.