Here is how the cart is cleared for the current session user:

$cart = Mage::getSingleton('checkout/cart'); 
$quoteItems = Mage::getSingleton('checkout/session')
                  ->getQuote()
                  ->getItemsCollection();
 
foreach( $quoteItems as $item ){
    $cart->removeItem( $item->getId() );    
}
$cart->save();

My question is how can i clear cart by user id, not by session?