There is no way to disable WYSIWYG (in the current version of Drupal Cloud) by default, even though the option appears under account settings. The workaround is to create a new Text format:
1. Go to Configuration (top menu)
2. Go to Text formats in the Content Authoring section.
3. Go to + Add text format
4. Give your new text format a name.
5. Select the roles allowed to access it.
6. Check the filters you want. You may want to just copy the Full HTML filters, which are: "Convert line breaks into HTML" - "Convert URLs into links" - "Correct faulty and chopped off HTML".
7. Save the new format.
8. If you want your new Text format to be the default format when you edit and create nodes, make sure it's at the top of the list on the Text formats screen. Otherwise, move it down and Save changes.
Note: If you're not the only site editor, you will need to consider how this change will impact others. Or maybe you're all aficionados of pure HTML editing, in which case, go you. :)
If anyone knows of a way to disable WYSIWYG using a more elegant method, please do let me know.