Realistically, especially if this was Office 365 with DirSync, the easiest way would be to leave the publicDelegates
attribute empty in the AD.
Make sure to use the UPN for the identity (which isn't required if working with a single mailbox):
Set-Mailbox -identity xxx@something.com -grantSendOnBehalfTo $null
or
Set-Mailbox xxx@something.com -grantSendOnBehalfTo $null