Purchase from one Store Only

Can you give an example?

Hey Edward, use this code (put in functions.php):

add_filter( 'woocommerce_add_cart_item_data', 'woo_custom_add_to_cart' );

function woo_custom_add_to_cart( $cart_item_data ) {

    global $woocommerce;
    $woocommerce->cart->empty_cart();

    // Do nothing with the data and return
    return $cart_item_data;
}

It allows the cart to only be updated with 1 product at a time, therefore meaning that a user can only purchase from one store at a time.