Why Can't I Delete my Subscribers?
If you cannot delete a subscriber, it means they're probably added to the WordPress Users list or the WooCommerce Customers list.
WordPress Users should be deleted as a User by going to the WordPress dashboard > Users page.
Customers added to the WooCommerce Customers list can not be removed from this list or from the MailPoet plugin, unless you delete their order from the WooCommerce plugin.
They can be marked as "Unsubscribed" though.