Step :1 Creat a custom attribute of boolean type which will be used to block customer login.

<?php

declare(strict_types=1);

namespace VendorModuleSetupPatchData;

use MagentoCustomerModelCustomer;
use MagentoCustomerSetupCustomerSetup;
use MagentoCustomerSetupCustomerSetupFactory;
use MagentoEavModelEntityAttributeSet;
use MagentoEavModelEntityAttributeSetFactory;
use MagentoFrameworkSetupModuleDataSetupInterface;
use MagentoFrameworkSetupPatchDataPatchInterface;
use MagentoFrameworkSetupPatchPatchRevertableInterface;

class AddBlockedCustomerAttribute implements DataPatchInterface, PatchRevertableInterface
{

    /**
     * @var ModuleDataSetupInterface
     */
    private $moduleDataSetup;
    /**
     * @var CustomerSetup
     */
    private $customerSetupFactory;
    /**
     * @var SetFactory
     */
    private $attributeSetFactory;

    /**
     * Constructor
     *
     * @param ModuleDataSetupInterface $moduleDataSetup
     * @param CustomerSetupFactory $customerSetupFactory
     * @param SetFactory $attributeSetFactory
     */
    public function __construct(
        ModuleDataSetupInterface $moduleDataSetup,
        CustomerSetupFactory $customerSetupFactory,
        SetFactory $attributeSetFactory
    ) {
        $this->moduleDataSetup = $moduleDataSetup;
        $this->customerSetupFactory = $customerSetupFactory;
        $this->attributeSetFactory = $attributeSetFactory;
    }

    /**
     * {@inheritdoc}
     */
    public function apply()
    {
        $this->moduleDataSetup->getConnection()->startSetup();
        /** @var CustomerSetup $customerSetup */
        $customerSetup = $this->customerSetupFactory->create(['setup' => $this->moduleDataSetup]);
        $customerEntity = $customerSetup->getEavConfig()->getEntityType(Customer::ENTITY);
        $attributeSetId = $customerEntity->getDefaultAttributeSetId();
        
        /** @var $attributeSet Set */
        $attributeSet = $this->attributeSetFactory->create();
        $attributeGroupId = $attributeSet->getDefaultGroupId($attributeSetId);
        
        $customerSetup->addAttribute(
            Customer::ENTITY,
            'blocked',
            [
                'label' => 'Blocked',
                'input' => 'boolean',
                'type' => 'static',
                'source' => '',
                'required' => false,
                'position' => 333,
                'visible' => true,
                'system' => false,
                'is_used_in_grid' => true,
                'is_visible_in_grid' => true,
                'is_filterable_in_grid' => true,
                'is_searchable_in_grid' => false,
                'backend' => ''
            ]
        );
        
        $attribute = $customerSetup->getEavConfig()->getAttribute(Customer::ENTITY, 'blocked');
        $attribute->addData([
            'used_in_forms' => [
                'adminhtml_customer',
                'adminhtml_checkout',
                'customer_account_create',
                'customer_account_edit'
            ]
        ]);
        $attribute->addData([
            'attribute_set_id' => $attributeSetId,
            'attribute_group_id' => $attributeGroupId
        
        ]);
        $attribute->save();

        $this->moduleDataSetup->getConnection()->endSetup();
    }

    public function revert()
    {
        $this->moduleDataSetup->getConnection()->startSetup();
        /** @var CustomerSetup $customerSetup */
        $customerSetup = $this->customerSetupFactory->create(['setup' => $this->moduleDataSetup]);
        $customerSetup->removeAttribute(MagentoCustomerModelCustomer::ENTITY, 'blocked');

        $this->moduleDataSetup->getConnection()->endSetup();
    }

    /**
     * {@inheritdoc}
     */
    public function getAliases()
    {
        return [];
    }

    /**
     * {@inheritdoc}
     */
    public static function getDependencies()
    {
        return [
        
        ];
    }
}

<?php

declare(strict_types=1);

namespace VendorModuleObserverFrontendCustomer;

class Login implements MagentoFrameworkEventObserverInterface
{

    /**
     * @var MagentoFrameworkAppResponseFactory
     */
    private $responseFactory;

    /**
     * @var MagentoFrameworkUrlInterface
     */

    private $url;

    /**
     * @var MagentoCustomerModelSessionFactory
     */

    private $customerSession;

    /**
     * @var MagentoFrameworkMessageManagerInterface
     */

    private $messageManager;

    /** 
     * @var MagentoCustomerApiCustomerRepositoryInterface 
     */
    private $customerRepositoryInterface;

    /**
     * Observer Constructor
     *
     * @param MagentoFrameworkAppResponseFactory $responseFactory
     * @param MagentoFrameworkUrlInterface $url
     * @param MagentoCustomerModelSession $customerSession
     * @param MagentoFrameworkMessageManagerInterface $messageManager
     * @param MagentoCustomerApiCustomerRepositoryInterface $customerRepositoryInterface
     */
    public function __construct(
        MagentoFrameworkAppResponseFactory $responseFactory,
        MagentoFrameworkUrlInterface $url,
        MagentoCustomerModelSession $customerSession,
        MagentoFrameworkMessageManagerInterface $messageManager,
        MagentoCustomerApiCustomerRepositoryInterface $customerRepositoryInterface
    ) {
        $this->responseFactory = $responseFactory;
        $this->url = $url;
        $this->customerSession= $customerSession;
        $this->messageManager = $messageManager;
        $this->customerRepositoryInterface = $customerRepositoryInterface;
    }
    /**
     * Execute observer
     *
     * @param MagentoFrameworkEventObserver $observer
     * @return void
     */
    public function execute(
        MagentoFrameworkEventObserver $observer
    ) {
        $customer = $observer->getEvent()->getCustomer();
        $blocked = $customer->getData('blocked');       
        if($blocked==1){
            $this->customerSession->logout();
            $this->messageManager->addErrorMessage(__('Your account has been blocked'));
            $redirectionUrl = $this->url->getUrl('customer/account/login');
            $this->responseFactory->create()->setRedirect($redirectionUrl)->sendResponse();
            return $this;
        }
    }
}