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As far as I know, on August 1, 2024, Microsoft is retiring the Office 365 A1 Plus plan. For the following steps to unassign a license, refer to the following steps.
Assign or unassign licenses for users in the Microsoft 365 admin center.
To block receiving users' emails in Outlook, you can refer to this article to set the Block/Receive mail list: Block or unblock senders in Outlook.
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