SELF-proclaimed sushi sufferer Jeremy Piven has slapped the Public Theater with a “cease and desist” letter over “The Piven Monologues,” a comic look at the star’s case of mercury poisoning and subsequent pullout from Broadway’s “Speed-the-Plow.” The show was staged last night and another performance is set for next month. Says Piven attorney Marty Singer, “We didn’t say you cannot do the play, we said you can’t make defamatory statements about our client.” An arbitrator ruled last month that Piven did not break his contract when he bailed. A theater rep declined to comment.