See
How Can I Remove Group Members Whose User Accounts Reside in a Specific OU?
http://www.microsoft.com/technet/scriptcenter/resources/qanda/jan08/hey0102.mspx

Below is probably the important part... just not sure how the user/array part would go without some testing

 Code:
$ADS_PROPERTY_DELETE=4
$objGroup.PutEx $ADS_PROPERTY_DELETE, "member", array(strUser) 
$objGroup.SetInfo