I was surprised when I found out there wasn't a way to stop the monthly payment other than switching and paying for a different "pay as you go" plan. I haven't encountered this sort of practice in any other decent web service, Ever. I contacted support and this was Mailchimp's response:
http://pastebin.com/raw.php?i=u1Frh4BJ
Mirror: http://pastie.org/6514771
(tl;dr you just have to pay a penalty if you want to stop for a while or spend a whole day unsubscribing 1500+ users to get it to 2000 subscribers)