Turn around in your seat and make direct eye contact with the people who are causing the disturbance. Most folks will be a little embarrassed, perhaps whisper a hasty apology, and stop the annoyance. A few will respond more aggressively ("What are you looking at, buddy?")
Ask the offenders to be quiet. Even though their behavior is rude, make sure your request is polite. ("I'm sorry, but I really can't hear the movie. Could you take your cell phone outside the theater to talk?")
Change seats. Sure, it would be more satisfying to make the rude movie-goer behave, but discretion is the better part of valor. You don't want to end up in a physical confrontation with someone who may be intoxicated, mentally ill or merely antisocial. If the obnoxious theater-goer ignores your request or responds to you with anger, finding a different seat where you can watch the rest of the movie in peace may be the smartest thing to do.
Report the behavior to the manager on duty. If the person is so loud or disruptive that even changing seats does not improve your situation, find one of the ushers and ask to speak with the manager on duty. The manager may elect to remove the people causing problems from the theater.
Leave. If the obnoxious movie-goer follows you to another part of the theater or otherwise seems fixated on you, leave the theater, alert the manager about what is going on and request an escort to your car.