You can stop them from logging in, but you can't stop them from trying. If they are pounding the server, you can reduce the auto ban settings to ban the IP sooner but that increases the chance that a legitimate visitor will be banned. With no front end login for your site, that is not a real concern.
You have done what you can, you now have to wait for them to get bored with beating their heads against your wall and go away.
Dale L. Brackin
Support Specialist
English: native
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!
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My time zone is
EST (UTC -5) (click here to see my current time in Philadelphia, PA)