1. go to your control panel
2. change your email address
3. do not confirm the login when your confirmation mail arrives
or in step 2, put in a fake email address so that you never get the confirmation email.
Either of these steps will close your tt account after 10 days.
After any of these measures, your login name on all your posts will look like margret's here in the screenshot.

Your posts will always remain part of TT.
If you ever want to come back, feel free! Just register a new user name.