Im not sure if this will work, but it has to do with permissions. If you are trying to block a whole drive, it doesn't matter what drive contains the OS.
1.Goto Finder
2.Right click drive you want "blocked"
3.Click Get Info
4.Click lock at bottom right
5.Enter User info of an Admin account
6.Press the + symbol in the bottom left corner
7.Select the "Guest" account under users and groups
8.Click select
9.Click the "Privilege" of the Guest account and set it to No Access
10.Now click the "Gear" icon to the right of the + symbol and select "Apply to Enclosed Items"
11.Click the lock
12.Exit the Get Info window
This should work. If the Hard drive that you are "blocking" contains the application folder, this could be a problem. In this case, skip step 10. Good luck and please report your findings.