Through my experiences, the best thing to do with a child that age is to say " No, Gentle", and touch their arm or hand softly, then show him what to do with the object in hand (move the truck back and forth, or stack the blocks). In order for this to work, it has to be done the same way each time. This technique usually takes about 2 weeks of practicing before you see any results. Just remember to be patient each and every time, and remember that he doesn't know that he is doing something "wrong", but is just testing out "action and reaction".

P.S Try not to show any sort of animated face (anger or laugh) that only encourages them.

Good Luck

Try stopping him before he throws a toy, remove it from his hand and tell him firmly NO THATS NAUGHTY! The tone of your voice will get his attention, and he will understand from that tone that he is doing something that displeases you! It may take a bit of time, because children like to test their parents to see how far they can push! Be persistant and continue to remove the object he wishes to throw while firmly telling him NO THATS NAUGTHY ! He does understand what you are saying!