If you are looking to remove Flume of all your account information, simply logout of an account, or remove it. This will remove any traces of your account information from Flume's local database and storage.
If you wish to completely reset Flume, including resetting all Preferences, Flume Pro status, caches and account information, hold down the
⌥ (OPTION) key as soon as you launch Flume. You will be asked to confirm that you wish to reset Flume.