Martin Sheen of "The West Wing" and "Late Night" host Conan O'Brien have been prohibited by their network, NBC, from participating in the celebrity edition of rival ABC's top-rated "Who Wants to Be a Millionaire." Variety reports that the stars, who would have been playing for charity, had agreed to face host Regis Philbin but were forced to withdraw once their bosses suggested that it might not be in NBC's best interest. "We would just prefer that our talent not compete against us during a sweeps month," a network spokesperson told the Hollywood trade paper, adding that the request was simply because of the ratings period and not a blanket prohibition against appearing at another time. Reps for CBS -- which reputedly also opposes its stars appearing on the show -- declined comment, as did a spokesperson from ABC. Last May, CBS star Ray Romano and FOX's David Duchovny were among those non-ABC stars to appear on "Millionaire."