When you are trying to enable/disable the AutoMapping feature on a user or a shared mailbox you can always use the Exchange management shell cmdlet:
The correct syntax would be something like this:
Add-MailboxPermission -Identity User1 -User User2 -AccessRights FullAccess -InheritanceType All -AutoMapping $false
The AutoMapping feature is stored in AD under the attribute msExchDelegateListLink. You can find all the AD user accounts that have AutoMapping enabled by checking the msExchDelegateListLink attribute.
In the rare case where you cannot seem to disable the AutoMapping using the Add-MailboxPermission cmdlet you can always delete the user account from the msExchDelegateListLink attribute.
If you just want to list the users that have the AutoMapping feature enabled for a certain mailbox, you can use this AD powershell cmdlet:
Get-ADUser "User1" -Properties msExchDelegateListLink | Select Name, msExchDelegateListLink