There is no way for you to control which toolbar buttons are displayed. Even if you could, that would not be nearly good enough. If a user technically has access to view a document, and know their way around Notes, they can view it, even if you hide all the navigation controls and put logic on the form to prevent it.
To keep documents secret from users, there are really only three secure ways:
- Deny them access to the server or database where the documents are stored (not useful in your case)
- Use Readers fields to control who has access to individual documents
- Use encryption to hide certain fields from all users except those who are issued the right key.