If you'd like to cancel, here's how you do it:
Go to the server list page by clicking on Servers at the top. Then click on Manage for your server, and click on Destroy on the right side button bar. Then wait about a minute while the server is destroyed.
Click on Account at the top, and then click on [Remove Card] to remove your card from our system.
.. and that's it! We'll never charge you again after that. If you ever want to come back, just run that process in reverse by adding a new card and then adding a new server.