How to deregister Stockholm Theme on a site that no longer exists?
If you don’t have access of old website wp-admin where the theme registered/activated then you have to contact theme author and request them to unregister/deactivate the theme license from the old domian/website.
contact theme Author and let them know How to Contact your purchased theme Author, theme author will be happy to assist you.
ありがとうございます！ テーマ作成者に連絡しました。