How can I disable mdi form menu item except one menu item?

Question

How can I disable all the menu item in mdi form except only one menu item.  In that menu item I can click and open a login form after I enter the username and password I will click the login button. Here I can open the mdi form with all menu items are enabled . How can I do this in my project . Please help me ... Here am I using c# windows application.