If you click edit a post you've made then once you're in edit click delete post. I think that works.
That may only delete your post, not the whole thread you started.
As far as deleting account, try the edit profile and see what you can do.
Might I ask, why do you want to delete the thread?
The easiest way to deal with your account is to simply edit all your posts to **********. I believe that the site requires at least 10 characters to constitute a post. Doing this will remove your presence here and then simply never return. Your account will eventually be pruned, though that will take years.
What user data?
The NSA knows who you are anyway.
This may help a little
Click on "Settings" uper right just below blue bar
click on "General Settings"
Check Private Messaging off
Don't receive Email Notification of new Private Messages
Visitor Messaging off
Under My Account edit password and e-mail and type in an invalid e-mail address.
I hope that helps.
I wonder what would happen if you just abandon it and did nothing.
Will It go dormant and eventually dead?