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

/*
	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)
{
        //Level ID of membership that you will allow level cancellation on. (Assume this is a free level or lower level than level ID 2 - see line 26)
	$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 2 instead.
		pmpro_changeMembershipLevel(2, $user_id);
	}
}
add_action("pmpro_after_change_membership_level", "my_pmpro_after_change_membership_level_default_level", 10, 2);