I can see where discussion sites don't allow for deletion as it is a royal PITA to maintain site integrity, threads, etc. if a user disappears.
It should be easy. Since all the posts should be in a database, just replace the content with something like --self-deleted-- and keep everything else the same.
For anyone quoting it from before it was deleted I'd say "fair use" if they're in the USofA.
Take your account:
Leave the user number the same (34294) and just --self-deleted-- the user name (chill) and anything you've listed in your profile.
And your post (#45807903) would also show --self-deleted-- but it would still show and my reply would still show in this thread.
And just make sure that no one else can ever use "chill" as a username or the email address you've used. That's just a different list.