mypmpro-lock-existing-members.php

<?php //do not copy
/**
 * This recipe lock members into their existing membership. 
 * They won't be able to cancel or sign up for a different membership level. 
 *
 * Change lines 21 and 45 to the level ID that requires you to be locked in. 
 *
 * If you don't want to prevent them from cancelling, comment out line 30 with //
 * 
 * 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/
 */

/**
 * Prevent members from cancelling their existing subscription
 */
function my_pmpro_free_remove_cancel_link( $pmpro_member_action_links ) {

	if ( pmpro_hasMembershipLevel( '9' ) ) { //Change the your chosen level ID

		unset( $pmpro_member_action_links['cancel'] );

	}

	return $pmpro_member_action_links;

}
add_filter( 'pmpro_member_action_links', 'my_pmpro_free_remove_cancel_link', 10);

/**
 * Check if they're trying to sign up for a different level other than the existing level they have
 */
function pmproml_pmpro_registration_date_checks( $value ) {

	global $pmpro_msg, $pmpro_msgt;

	if( !empty( $_REQUEST['level'] ) ){
		$level_id = intval( $_REQUEST['level'] );
	} else {
		$level_id = 0;
	}

	if ( pmpro_hasMembershipLevel( '9' ) && $level_id !== 9 ) { // Change to your chosen level ID

		$pmpro_msg = __( "Registration has been disabled for any level other than your existing level. ", 'paid-memberships-pro' );
			
		$pmpro_msgt = "pmpro_error";
		
		return false;

	}

	return $value;
}
add_filter( 'pmpro_registration_checks', 'pmproml_pmpro_registration_date_checks', 10, 1 );