If you just unsubscribe that user, the user can re-subscribe again and receive your emails. I suggest creating a new segment called "Blacklisted" or similar and add unwanted users to that segment (not list, but segment). When you want to send a new email blast you can set to send it to all users except for the Segment name "Blacklisted".


You can go on Listst/ Manage Contacts/ Click unsubscribe - That takes this Email / contact to the unsubscribe list which is what you are looking for. Mailchimp won't send emails to unsubscribed emails


Contact your ISP and see if they can/will do anything about it. Buy a domain from a registrar that provides email services that include a mail server on which you can do sender address verification (either you have full admin control over the SMTP server and can install / configure as you wish, or their provided solution allows you, or them, to set up sender ...

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