You can setup a cron job that will delete all users that are not validated within 24 hours.

For domain throttling, just register a hook for 'action', 'register' and deny if email is in the wrong domain.

    // Delete all user accounts that have been disabled by uservalidationbyemail plugin
    // that were created more than 24 hours ago

    elgg_register_plugin_hook_handler('cron', 'hourly', function() {

        $ha = access_get_show_hidden_status();
        access_show_hidden_entities(true);
        $ia = elgg_set_ignore_access();

        $time = time() - 24 * 60 * 60; // 24 hours ago
        $users = elgg_get_entities_from_metadata([
            'types' => 'user',
            'metadata_name_values_pairs' => [
                'name' => 'disable_reason',
                'value' => 'uservalidationbyemail_new_user',
            ],
            'wheres' => [
                "e.time_created <= $time",
                "e.enabled = 'no'",
            ],
            'limit' => 0,
            'batch' => true,
        ]);

        $users->setIncrementOffset(false);

        foreach ($users as $user) {
            $user->delete();
        }

        access_show_hidden_entities($ha);
        elgg_set_ignore_access($ia);
    });

    // Set early priority, so this is triggered before uservalidationbyemail kicks in
    elgg_register_plugin_hook_handler('register', 'user', function($hook, $type, $return, $params) {
        
        if ($return === false) {
            // something else prevented registration
            return;
        }
        
        $user = elgg_extract('user', $params);
        if (!$user) {
            return;
        }
        
        $email = $user->email;
        list($prefix, $domain) = explode('@', $email, 2);
        
        $blacklisted_domains = [
            'yahoo.com',
        ];
        
        if (in_array($domain, $blacklisted_domains)) {
            register_error("Emails from $domain domain are not allowed to register");
            return false;
        }
        
    }, 100);

In the future, do not use pages to ask question. Go to a respective group and start a discussion. One question - one discussion.

Thank you ismail for your reply .. Oh sorry my mistake i thought it is a discussion i am opening bad eye ..

Take a look at https://elgg.org/plugins/1422171 regarding restrictions on allowed email domains.

The hook used in Daniel's link is more suitable for email validation, but the approach is the same.