There's no option to delete your account from the system even the mods can't help you. the only option is to abandon it and move on.
It's not possible. Just abandon it.
Do this: if you want your record to non exist on forum, Delete all the deletable post and edit the non deletable ones to trash and move them to archive board. Then change your password to something you can't remember, also change your email to an invalid email then abandon your account. That way you can't reactive your account and it'll be forgotten.
Second option: Get yourself banned.