how can I delete my account ?

Do you want to delete/cancel your entire Launchpad account? If so, please follow this FAQ: https://answers.launchpad.net/launchpad/+faq/47

If you want to keep your Launchpad account but remove all your files and machines from your Ubuntu One account, then you can do the following:

Remove files
1. Delete all files from your ~/Ubuntu One folder
2. Allow the client to synchronize the deletions
3. Check https://one.ubuntu.com/files/ to be sure no files exist

Remove machines
1. Go to https://one.ubuntu.com/account/machines/
2. Select the check box next to all the machines on the account
3. Click the "Remove selected computers" button