If you just want to stop monthly billing, you don't have to permanently close your account. You can stop monthly billing by purchasing a Pay As You Go plan to switch your billing type and stop monthly auto renewal charges. You can purchase a pay as you go plan under Account > Upgrade Plan or Buy Credits.
If you want to close your account altogether, we hate to see you go, but you can close your MailChimp account by logging in and visiting the Account > close my account.
This action cannot be undone.