1. Open the EAC
Click on start and then expand Exchange Server 2016 and click on Exchange Admin Center.
2. Login to the EAC
If you have a certificate warning, click yes to continue and then enter the credentials to login and press enter.
3. Click on Recipients
On the left hand side, click on Recipients.
4. Select a Mailbox
Select a mailbox you want to disable.
5. Select the ... option
Click on the 3 dots above the mailbox and then select disable.