TJ's right. Save an urgently expressed "NO!" or "STOP!" for the really important moments when your son MUST be protected from whatever he's about to do. Otherwise, I have found it works well to calmly state something like, "Please don't do that." "Let's do this instead; see how much fun we'll have!" "If you x, then y will happen." "If you want to throw, I need you to throw only soft toys." "Please put that down, right now." "You can do that two more times, and then it's time for lunch." (Notice the polite tone, also important if you want a polite child.)
And whatever you do, enforce your instruction immediately. Don't give your child a chance to develop the habit of ignoring you, or expecting that you won't follow through. It will require extra effort from you now, but I promise you, it will pay off. When you say no, or explain quietly why you don't want him to do something, be prepared to stop the action yourself, immediately and without drama, by removing your son from the temptation, or the temptation from your son.
I have found that there are hundreds of ways to say no that don't use that word, which kids will quickly learn to tune out if they hear it constantly, especially if you are slow to enforce it. Use an attractive distraction when you can.
And pick your demands carefully. He also needs a little leeway to learn the consequences of dropping, spilling, falling, etc., for your explanations to have some real meaning. Giving him some opportunity to do what he wants in a guided, playful way will cut down his frustration and give him a chance to do what he needs to do: explore and discover. Becky L's ideas are right on target.
Your child will persist in doing whatever he is curious about. He's just a normal little guy, and that persistence will serve him well as he learns how his body, his family, and the world work. And its your job, mama, to help guide his attitudes, expectations and behavior with love, consistency, and understanding. And tremendous patience and persistence of your own.