Simply create and use a smart collection for unused media, then enable the filter pop-up menu for Hide Rejected.
To create smart collection:
(1) File>New>Smart Collection
(2) Name it "Unused Media"
(3) Double-click on the new smart collection to open it
(4) Press plus key to add a filter , pick "Used Media"
(5) Click the filter to change it to "Unused".
Whenever you select the smart collection for "Unused Media" and the pop-up menu for Hide Rejected, it will filter on them both.