One or more folders in Outlook must not be accessible by other users.
The safest way to keep your data private is to not share the folder from Outlook. The sharing of Outlook folders is configured within the sharing Outlook client. Click Tools, Share, Settings and select which folders you wish to share or unshare. Any folders that are not selected will not be shared up to the WorkgroupShare server.
Note that sharing a folder to the server does not mean that others can access the data. Once a folder has been shared, access permissions must be set before others can view the shared folder. Access can be set in the WorkgroupShare Administrator and in the case of WorkgroupShare v2 the access permissions can also be configured in the client (Tools, Share Settings).
By default, all shared Outlook folders have no users in the access list and so No Access is set by default on newly shared folders. By adding users to the access list, the resource then becomes accessible with the permissions as set. If an Outlook folder has no users listed in its access list then it is only shared as far as the WorkgroupShare server. This may be necessary when sharing a Calendar folder where other users will need to see free time when scheduling meetings but must not have access to the data in the folder.
To set No Access on a shared folder you just need to remove all names from the access list. If a name does not appear in the access permissions list then it is implied to have No Access permissions.