How do you delete books from compromised account?

One of my clients called to complain that hundreds of adult books showed up in their iBooks purchased list. I suspect the client's iTunes account password was compromised and someone else purchased the free books on the account.

I've done some searching, and found two promising articles, but nothing on actually removing the titles from the client's account.

https://support.apple.com/en-us/HT204343 provides guidance on deleting content from your iphone.

https://support.apple.com/en-gb/HT201322 provides instructions on using iTunes to hide purchased content from your account.

Does anyone know if an ibook can be permanently deleted from an account?