Ultimate Member customization. Block registration that contains specific strings in the email domain.

um_validate_email_domain.php

<?php

// Custom validation for the "E-mail Address" field on registration.
// Block registration that contains specific strings in the email domain.
function um_validate_email_domain( $args ) {

	// List blocked email domains here (entire domain or a part of it).
	$blocked_email_domains = array(
		'vovk.store'
	);

	// Change error message here.
	$message = __( 'You can not use this email domain for registration', 'ultimate-member' );

	if ( isset( $args['user_email'] ) && is_email( $args['user_email'] ) ) {
		$email_domain = array_pop( explode( '@', trim( $args['user_email'] ) ) );
		foreach ( $blocked_email_domains as $blocked_email_domain ) {
			if ( false !== stripos( $email_domain, $blocked_email_domain ) ) {
				UM()->form()->add_error( 'user_email', $message );
			}
		}
	}
}
add_action( 'um_submit_form_errors_hook__registration', 'um_validate_email_domain', 20 );

This code snippet can be useful if spammers are using multiple email domains based on the same higher-level domain.

Use the "Blocked Email Addresses" setting on wp-admin > Ultimate Member > Settings > Access > Other to block an entire email domain. Read article Block Email address on Registration for details.