I want to stop using Ubuntu-One. How do I delete whatever is stored in the cloud?
Just move all your files from your synced local folders to a not synced folder, and the files will be deleted from the cloud too (give it some time to sync). You can check on the web interface to go for sure that all files are deleted: https://one.ubuntu.com/files/
You can also use the web interface to delete your files from the cloud (be sure that you have backup somewhere else).
You can stop syncing a local folder by right clicking on it in Nautilus (the default file manager), and choose the Ubuntu One -> Stop Synchronizing This Folder option. You can also use the web interface to stop sychronizing a folder: just click on the more button and choose the Stop syncing this folder option.
You can disable the file sync on a machine completely by starting the Ubuntu One Control Panel and remove the tick from the Enable File Sync option under the Services tab.
After you have removed all your files and turned of the file syncing, you can remove your devices on the Account page of the web interface: https://one.ubuntu.com/account/
It's simple, Just find your Ubuntu One folder in you Home folder, open it and delete everything it it. Vise versa just copy and past files into that folder to add files into you cloud. Simple.