If you post to #StackExchange, you license that content to them under CC-BY-SA. You can't revoke that license, so you can't remove that content.
It shouldn't surprise you that they stop you from trying. Imagine a Wikipedia editor trying to remove their edits.
However, what it does give you is the right to host all of SE elsewhere. To fork it, if SE violates its contract with the community. This is how SE was always designed to work. From an interview with Spolsky:
