Light box limit reached - How do I delete

I am trying to add some light boxes but I am getting an error saying Light Box limit is reached.

I can see there are 10 lightboxes - but they are not in use. I have deleted them - and I cannot view them in the contents bar - however the still show in the light boxes tab.

Have can I delete these so I can add some new Light boxes?

any help would be great

thanks