Resources can be hidden from some or all users via a variety of means.
Hiding Resources from Non-Site Members
If your site is not completely private and/or is a member of any publicly accessible portals, then your resources could be viewed and searched for by anybody, regardless of their membership status to your site.
In order to hide specific resources from non-site members, visit the Visibility tab under Properties when editing your resource and set the Resource is Publicly Visible option to No.
Hiding Resources from Specific Site Members
Once a user or group of users are no longer able to reserve resource, you can remove visibility of this resource by visiting the Visibility tab under Properties when editing your resource and setting the Resource Hidden to Users Who Cannot Reserve option to Yes.