Hi @Glenn Maxwell , agree with the suggestions above, and it should be the easiest way to meet your needs.
The test result here:
Locate to O365 Admin center, Users, Active users, choose the user you want to deny access, block sign-in
After you block this user, he/she will not be able to login the account through OWA, outlook and any other ways
However, other people who has full access permission will see new mails sent to his mailbox as well:
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