This isn't a request we get very often, most people simply stop using an account if they no longer wish to participate for whatever reason.
We've had a couple of people in the past who felt the site was providing too great a distraction/time-sink, and that they lacked the will-power to simply stop visiting. In those cases moderators have simply suspended the account; temporarily in the case of someone who just wanted to get through an exam period un-distracted, and permanently in at least one other case I can recall.
If you actually want your details scrubbed, you'll need a member of staff to help you. I can only recall one prior instance of this being done, and in that case the actions taken were:
- Change the username.
- Remove the profile information ("About me" and contact fields) and forum signature.
- Scramble the email address to a nonsense, non-existent address.
Note that as a regular user you can actually make the majority of these changes yourself. This would leave your posts intact but displaying a different obviously unusual username, and although the presence of quotes in any topics you've participated in may give away the previous username no other personal information would remain unless you happened to include something in a post body.
As a general rule, we don't allow the removal of post contents without a compelling reason to do so -- candidates for removal may include offensive materials, spam, or materials involved in a legal dispute -- as this is damaging to our community, and really isn't fair on other users. If you had one or more specific posts you wanted removed you could always ask and provide a reason you feel they should be removed, but the answer may well be "no". As noted above, there are also plenty of archive sites out there that we have no control over, and your content would generally remain accessible there anyway.
If you're just curious I hope that answers your question. If you're actually asking because you're considering closing your account, shoot me a private message -- but we'd be sad to see you go!