How to exclude an OU during a search?

I have this code that works to move old computer accounts to an inactive OU.  

$Inactive_computer_OU = 'OU=InactiveComputers,DC=Acme,DC=com'

$sourceOU = 'Acme.com/WorkstationComputers'

Set-QADPSSnapinSettings -DefaultSizeLimit 0

$old = (Get-Date).AddDays(-45).ToFileTime()

$ldapFilter = '(pwdLastSet<={0})' -f $old

Get-QADComputer -LdapFilter $ldapFilter -SearchRoot $sourceOU -IncludedProperties pwdLastSet -SizeLimit 9000 | Move-QADObject -NewParentContainer $Inactive_computer_OU

But under $sourceOU = 'Acme.com/WorkstationComputers' I have a sub-OU located here:

'Acme.com/WorkstationComputers/XenDesktops'

I need to exclude this 1 OU from what this script does.  How do you handle that?