This recipe removes the "Join Now" registration link on the PMPro login page.

my-pmpro-login-forms-handler-nav.php

<?php

/**
 * This recipe removes the "Join Now" registration link on the login page.
 *
 * This link is shown only when the "Membership  [ ] Anyone can register" option was set
 * on the WordPress dashboard under Settings > General. To remove the "Join Now" link
 * may also be achieved through deactivating this option if all users are required
 * to be a member.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_login_forms_handler_nav( $links ) {
	global $pmpro_pages;

	// bail if not on login page.
	if ( ! is_page( $pmpro_pages['login'] ) ) {
		return $links;
	}

	unset( $links['register'] );
	return $links;
}
add_filter( 'pmpro_login_forms_handler_nav', 'my_pmpro_login_forms_handler_nav' );