Revoked Permissions

Note: You can revoke a specific permission only if you can do so at the level that the permission was first granted. That is, if a permission was granted to a user as a result of inheritance from a higher level, you cannot revoke that permission unless you can also access and modify the higher level.
If User B would like to revoke User A’s access to Folder B, he cannot do so, because he has no permissions to Folder A, where User A’s access was original granted. Thus, User B cannot block User A from Folder B.