The default tabs in the customer edit menu are added in the core file
app/code/core/Mage/Adminhtml/Block/Customer/Edit/Tabs.php
To remove the "Shopping Cart" tab, you need to extend Mage_Adminhtml_Block_Customer_Edit_Tabs and override the _beforeToHtml() function without this portion of code:
$this->addTab('cart', array( 'label' => Mage::helper('customer')->__('Shopping Cart'), 'class' => 'ajax', 'url' => $this->getUrl('*/*/carts', array('_current' => true)), ));