He's not acting out. Your pitcher-in-training's predilection for tossing is not an act of defiance or aggression. Sometimes it's your preverbal toddler's only means of communication. An empty sippy cup thrown on the floor could mean he's still thirsty and wants more. Become attuned to what's being thrown and you might learn to better understand his needs. Still, insight into why he's throwing things doesn't help clean up the spills. Try to channel his energy elsewhere.
You can set limits. Tell him what may and may not be thrown (balls good, food bad), and where throwing is okay, preferably outdoors and not from his high chair. Gently say "No throwing," and shake your head firmly with a serious look on your face. If he persists, tell him: "If you want to throw, I'll take you out of the high chair and we'll go in the backyard and play catch." Then follow through. As long as you're calm and consistent, he will learn.