Top Hollywood talent firm Creative Artists Agency (CAA) is warning celebrities — including Robin Williams and Marisa Tomei — that it won’t work with them if they remain with a new management company founded by the agency’s one-time chief. CAA has told clients it will refuse services, for which it takes a hefty percentage of a star’s pay, if the actors remain with the Artists Management Group — created by former superagent and ex-CAA chief Michael Ovitz. “After having met with Ovitz on several occasions over the past several months to discuss the nature of our collaboration, he agreed not to raid our agents or clients, and he has violated his word,” said Richard Lovett, CAA president and partner. The message went to those jointly represented by CAA and Ovitz’s new firm, including directors Sydney Pollack and Martin Scorsese. Among CAA’s other big-name clients are Tom Hanks, Tom Cruise and Steven Spielberg. Ovitz was unavailable for comment, a spokeswoman said Tuesday.