Establish the rules, with a smile. Deadlines are real. So are the overall objectives of the web site. And so is your audience. These comprise the rules of the web design dance.
Content needs to be on time, on topic, and appropriate to the audience.
Know when to call the bouncer over. Some experts are straightforward and respectful. Others are passive aggressive and challenging. And some are just plain aggressive. They're always breaking the rules, but just barely, just enough to make your work life miserable.