The only way to free up space on a tape library is to Recycle media to remove protection. As long as the tape is in the correct media pool, it will be reused for some future backup
I would especially look to see if any media is permanently Protected. If you see any media in this condition, right-click, go to Properties, and click the Objects tab. Scroll all the way to the right, there is a column "Data Protection". For media to be reused, all objects should have a protection status of "none' or 'Expired'
If you do see media that is Permanently protected, I would advise that you check your backup specification that created this media (scroll back to the left), and see if it may have been left at Permanent protection (default) by mistake