From their site:
To terminate your DotCloud account you can:
Destroy all your applications with dotcloud destroy (Use dotcloud list to list them);
Uninstall DotCloud from your computer:
$ rm -rf ~/.dotcloud
$ sudo pip uninstall dotcloud
$ sudo pip uninstall dotcloud.cli
Finally, ask us to remove your account on http://support.dotcloud.com/.
So, if you have applications, you may want to delete these first. Sounds like quite a hassle!