If you wish to close your MessageMedia account, in the parent account go to Settings > Account, and click on the Settings tab. At the bottom you'll see an option to change or close your account.
Alternatively, you can contact firstname.lastname@example.org with your account number and reason for cancellation, or you can call us on 1800 155 228.
PLEASE NOTE - if you only want to remove a sub-account, you can do this in the web portal. See this article for instructions.