I took a hint from Lrrr’s answer and populated the drop-down list with only user-added websites, that is, “Please select” and “Admin” is no longer available as options by adding the following line:
$form->getElement('website_id')->setValues(Mage::getSingleton('adminhtml/system_store')->getWebsiteValuesForForm());
at the end of this function:
Mage_Adminhtml_Block_Customer_Edit_Tab_Account::initForm
The ideal way would, of course, be to override the above function in one native module, but in our case, overriding the above class creates a conflict for the other extension that we installed, so I did it this way.
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