Custom Block Write rules do not stop writes to UNC paths.
In the Bit9 console: Rules > Software Rules > Custom tab
Rule Type: Advanced
Write Action: Block
When this rule is applied, if a local path is entered under "Path Or File", the rule will work as intended. If this is a UNC path, no block will actually occur and writes will be allowed to this directory.
Cb Protection only tracks execute actions over a UNC path, but does not track write actions to a UNC path.
The best way to configure these network shares would be to utilize NTFS(Active Directory) and Sharing permissions, outside of Cb Protection