/**
 * @param string $email
 * @return bool
 */
function deleteCustomerQuote($email) {
    // vendormagentomodule-customeretcdi.xml : MagentoCustomerApiCustomerRepositoryInterface = MagentoCustomerModelResourceModelCustomerRepository
    // vendormagentomodule-quoteetcdi.xml : MagentoQuoteApiCartRepositoryInterface = MagentoQuoteModelQuoteRepository
    /** @var MagentoFrameworkAppObjectManager $objectManager */
    $objectManager = MagentoFrameworkAppObjectManager::getInstance();
    /** @var MagentoCustomerApiCustomerRepositoryInterface $customerRepository */
    $customerRepository = $objectManager->get('MagentoCustomerApiCustomerRepositoryInterface');
    /** @var MagentoQuoteApiCartRepositoryInterface $cartRepository */
    $cartRepository = $objectManager->get('MagentoQuoteApiCartRepositoryInterface');
    try {
        $customer = $customerRepository->get($email);
        $cart = $cartRepository->getForCustomer($customer->getId()); // MagentoQuoteApiDataCartInterface
        $cartRepository->delete($cart);
        return true;
    } catch (Exception $exception) {
        return false;
    }
}

/**
 * @param string $email
 * @return bool
 */
function deleteAllCustomerQuotes($email) {
    $objectManager = MagentoFrameworkAppObjectManager::getInstance();
    /** @var MagentoCustomerApiCustomerRepositoryInterface $customerRepository */
    $customerRepository = $objectManager->get('MagentoCustomerApiCustomerRepositoryInterface');
    /** @var MagentoQuoteModelResourceModelQuoteCollectionFactory $quoteCollectionFactory */
    $quoteCollectionFactory = $objectManager->get('MagentoQuoteModelResourceModelQuoteCollectionFactory');
    /** @var MagentoQuoteApiCartRepositoryInterface $cartRepository */
    $cartRepository = $objectManager->get('MagentoQuoteApiCartRepositoryInterface');
    try {
        $customer = $customerRepository->get($email);
        $customerId = $customer->getId();
        $quoteCollection = $quoteCollectionFactory->create()->addFieldToFilter('customer_id', $customerId);
        if ( $quoteCollection->getSize() ) {
            /** @var MagentoQuoteModelQuote $item */
            foreach ( $quoteCollection as $item ) {
                $cartRepository->delete($item);
            }
        }
        return true;
    } catch (Exception $exception) {
        return false;
    }
}