When something is posted anywhere in the SE network, the poster grants SE a non-revokable license (CC BY-SA 3). The appropriate action to take is roll back and tell the poster not to vandalize their own posts. Should they not listen and decide to vandalize it again (whether a rollback or a new edit), mod flag the post instead of ending up in a rollback war.
There's no specific need to mod flag the post the first time it's been vandalized, but if the user rolls back/re-vandalizes it. See this post on meta.SE:
There's not really any reason to flag a one-time occurrence, though. Vandalism of multiple posts, or continued rollbacks on the same post, will be automatically flagged for moderator attention by the Community user and action will be taken.
The terms of service also cover the licensing:
You agree that any and all content, [...] you provide to the public Network (collectively, "Subscriber Content"), is perpetually and irrevocably licensed to Stack Overflow on a worldwide, royalty-free, non-exclusive basis pursuant to Creative Commons licensing terms (CC-BY-SA), and you grant Stack Overflow the perpetual and irrevocable right and license to access, use, process, copy, distribute, export, display and to commercially exploit such Subscriber Content, even if such Subscriber Content has been contributed and subsequently removed by you as reasonably necessary to, [...]
If it's an attempt to delete, it's also a good idea to leave a comment telling the OP vandalizing the post is not the correct way to delete it. The exception here is if it's a question with upvoted answers; it can't be deleted by OP.
