1) In accordance with the site terms, once you post, the data becomes licensed through CC-BY-SA (in my limited understanding, "anyone can use it, but must link back to source and keep the license"). So once you give permission (by posting), it's technically not exclusively yours as in "I'm taking it away again" (I think because the page it's on might be seen as a derivative work, and you don't control those - due to the license you agreed to, SE (and anyone else) has a right to make derivative works, as long as it complies with the license terms).
That said, you could (usually) delete a post that you created, using the "delete" link; note that it's a soft-delete (i.e. hides it from public view, data stays in db). See also this: How does deleting work? What can cause a post to be deleted, and what does that actually mean? What are the criteria for deletion?
2) Uh, don't post sensitive data on publicly accessible sites (such as SO)? If you already did that, you need to email email@example.com with that question's details and ask that the data be deleted. If you give your reasons, the SO team might be able to help you; also, asking nicely goes a long way towards the team actually helping you :)
3) Deleting your account is possible - see this: How can I delete my account?