Stop users with an active membership from checking out for a membership.

my-pmpro-block-active-members-from-checkout.php

<?php
/**
 * Block active members from checking out but allow renewal if membership is expiring soon.
 *
 * 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/
 */

// Remove change link on account page.
function my_pmpro_account_links_remove_change( $pmpro_member_action_links ) {
	unset( $pmpro_member_action_links['change'] );
	return $pmpro_member_action_links;
}
add_filter( 'pmpro_member_action_links', 'my_pmpro_account_links_remove_change' );

// Restrict and stop checkout for active members.
function my_pmpro_block_existing_members_from_checkout( $okay ) {
	global $current_user, $pmpro_level;

	if ( ! $okay ) {
		return $okay;
	}

	if ( ! empty( $current_user->ID ) && pmpro_hasMembershipLevel() && ! pmpro_isLevelExpiringSoon( $pmpro_level->id ) ) {
		// disable checkout.
		pmpro_setMessage( 'You are already a member. Please cancel your active membership first.', 'pmpro_error' );
		return false;
	}

	return $okay;
}
add_filter( 'pmpro_registration_checks', 'my_pmpro_block_existing_members_from_checkout' );

// Warn existing members on checkout.
function my_pmpro_warn_existing_members_on_checkout() {
	global $current_user, $pmpro_level;

	// bail if user is not logged in or do not have a membership level
	if ( empty( $current_user->ID ) || ! pmpro_hasMembershipLevel() ) {
		return;
	}

	if ( ! empty( $current_user->ID ) && pmpro_hasMembershipLevel() && ! pmpro_isLevelExpiringSoon( $pmpro_level->id ) ) {
		pmpro_setMessage( 'You are already a member. Please cancel your active membership first.', 'pmpro_error' );
	}
}
add_action( 'pmpro_checkout_after_parameters_set', 'my_pmpro_block_existing_members_from_checkout' );