Place a PMPro member in another level when they cancel... unless they are cancelling from that level.

pmpro_after_change_membership_level_default_level.php

<?php
/**
 * When users cancel (are changed to membership level 0) we give them another "cancelled" level.
 * Can be used to downgrade someone to a free level when they cancel.
 * Will allow members to the "cancel level" to cancel from that though.
*/
function my_pmpro_after_change_membership_level_default_level( $level_id, $user_id ) {
       
	// Set this to the id of the level you want to give members when they cancel.
	$cancel_level_id = 1;

	// If we see this global set, then another gist is planning to give the user their level back.
	global $pmpro_next_payment_timestamp;
	if ( ! empty( $pmpro_next_payment_timestamp ) ) {
		return;
	}
	
	// Are they cancelling?
	if ( $level_id == 0 ) {
		// Check if they are cancelling from level $cancel_level_id.
		global $wpdb;
		$last_level_id = $wpdb->get_var( "SELECT membership_id FROM $wpdb->pmpro_memberships_users WHERE user_id = '" . $user_id . "' ORDER BY id DESC" );
		if( $last_level_id == $cancel_level_id ) {
			return;	// Let them cancel.
		}
		
		// Otherwise give them level $cancel_level_id instead.
		pmpro_changeMembershipLevel( $cancel_level_id, $user_id );
	}
}
add_action( 'pmpro_after_change_membership_level', 'my_pmpro_after_change_membership_level_default_level', 10, 2);