To Disable Outlook Auto-mapping from the Exchange Management Shell:
1. Make sure user1 is the account that you’re granting full permission
2. Make sure user2 is the account that is getting full permission.
3. Add-MailboxPermission -Identity user1 -User ‘user2’ -AccessRight FullAccess -InheritanceType All -Automapping $false
This will Grant the Full access of user1 to user2 and disable the Auto-mapping.