old user can still access mailbox after removing msexch-masteraccountsid

Hi,

I've changed the user mailbox and user by running the following command on the linked mailbox

Set-User -Identity "alias" -LinkedMasterAccount $null

Issue is that the previous user (the msexchmasteraccount) still has access, and yes, that user is still enabled. 

My question is - how can I remove that access to the mailbox? The msexchmasteraccount is empty in adsiedit.