First, you need to go to the email you used to set up that account and open up the original message from flyordie stating your username, passcode, ect. If you don't have that original email from flyordie, just simply log onto the account that you want to delete and change the passcode to any random characters. Then, log onto your email you set that certain account up with and click on the link to the message from flyordie. Scroll down to the bottom of the message and click a link that looks something sort of like this: "if you did not sign up this account, click here to invalidate your account... blah-blah-blah...". It should take you to a flyordie page in a new tab of your browser and it says you cannot sigh that persona up again.
Hope this has been helpful...
- Ghecko