One thing they could do is tell you to stop. If they have told you to stop, and taken measures to block you out (blocking crawlers besides google) then persisting is illegal. I believe somebody got succesfully sued by facebook for continuing to scrape after facebook told them to stop. I'm not sure about the legality if you haven't been explicitly asked to stop, but as long as you are never blatant enough for them to notice, you shouldn't have any trouble.