Just yesterday a colleague of mine asked me how to undo an Active Directory object property change from the value he erroneously put back to <not set>. It turned out that I never actually blogged about that – so here you go. 
Clearing AD attributes us actually as easy as just setting the value to $null. For example, here’s how you do it for properties which we have exposed in Set- cmdlets parameters:
Set-QADUser 'Amy Hardy' -City $null
Or for more internal attributes:
Set-QADUser 'Amy Hardy' -ObjectAttributes @{adminDescription=$null}
Hope that helps!
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Finally figured out how to do this without the Quest extensions.
$userobj = [ADSI]“LDAP://cn=myname.OU=People.DC=example,DC=com”
$userobj.put(“description”,@())
$userobj.SetInfo()
The empty array is necessary to clear the description without error, not sure why, but I believe it is because @() actually generates a null array object. $null does not work; it gives you errors.