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I would put such a statement in your automated welcome email. I.E. something to the effect of "By signing up for this site, you hereby agree to ....... ." I don't think it's something for everyone. *Hopefully* must sites don't need it. But, even if somebody does not have such a statement, they still have full control over the site and the ability to remove somebody anyway. Web sites are private things. They are controlled and paid for by somebody. Nobody has the right to post something on your site that is not in accordance with your rules. I've also seen some people make an ettiquette page and posts things like this there.