Purchased books will not be deleted from iBooks library

My youngest seems to have purchased some free books from the iBooks store with explicit contents. Now I have applied parental control over all of the devices yesterday to ban her from doing so again; but I would like to know how I can delete the already existing books from the purchased list; whenever I use my iBooks on my Mac in public it will show the book covers and it is really embarrassing.

P.S. I don't want to just hide them I want to permanently delete them.

Thank you!