Thank you for reaching out to Atlassian Community!
Currently, it’s not possible to mark an email as spam or block this address, however, there is a workaround that may help you if there are few addresses that you need to block.
If you are a site-admin, you can go to Administration > Jira Service Desk and instead of deleting, you need to deactivate the account (revoke access).
If we delete, if the project is open to customers to create their account, they can create the account again if they send a new email, but if we set as inactive, they won’t be able to create the account and create tickets, all new emails will be rejected by Service Desk informing that the user is not active.
We also have a feature request suggesting the implementation of a blacklist:
Hope this helps!
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