You can add a Terms of Service before a user signs up that they have to agree to. At least that way if they're not sure why you removed a post, you can say you agreed to the terms when you signed up. On the other hand, rules are pointless, no one will read them, they'll get ignored, then argued about when you try to enforce it. Just keep it simple and along the lines of "don't be an idiot." Hope this helps.