[LaDidi]

Have you remove access or DENY access to account ?
It's not the same result...

Have you tried LockFile or LockFileEx API ?
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/fileio/fs/lockfile.asp
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/fileio/fs/lockfileex.asp