So the idea is, that you can prepare an action batch without actually executing it. That's the idea behind the "confirmed" parameter. As long as that is set to false, the action batch will not be executed.
If you haven't set the confirmed to true after a week the action batch will be removed (this is likely to keep Meraki's action batch DB from cluttering up).
Once set to confirmed, you can no longer "cancel" it, so therefore you can't delete it, nor can you change it. I didn't find it documented, but I assume it will also automatically be deleted after a week. So there's no manual cleanup possible.
I also found the PUT command to refuse changing the actions part. But you can delete it and create a new one, does that help?