Document permissions are ultimately controlled by the project field. If both 'restricted' and 'unrestricted' documents are in the same project, we don't have the ability to individually select specific documents to be locked down.
We recommend protecting important document by placing them in a separate project. If desired, you create a sub-project within the same main project, and lock down this IM sub-project only.
Please let us know if you need any assistance setting this up.
