Removing quarantined devices when the mailbox attribute has been deleted

I have a few Exchange 2010 mobile devices in quarantine that belonged to employees that have been termintated and the mailbox attribute removed. when I try to remove the devices using;

Get-ActiveSyncDevice | Where {$_.DeviceAccessState -eq "Quarantined" -and $_.FirstSyncTime -lt (Get-Date).AddMonths(-6)} | Remove-ActiveSyncDevice -Confirm:$false

I get an error stating

Couldn't find '<AD domain>/<OU path><Username>,as a recipient.
   + CategoryInfo          : InvalidArgument: (:) [Remove-ActiveSyncDevice], RecipientNotFoundException
   + FullyQualifiedErrorId : 44C6758C,Microsoft.Exchange.Management.Tasks.RemoveMobileDevice

is there a way to remove the device after the mailbox has been removed?