DS1 loved throwing things too As some of the others have suggested, we took things away and gave him better alternatives like soft balls and kept reinforcing the rules like, we only throw balls, we only throw outside, we don't throw things at people unless they're ready to catch because it can hurt them etc etc. Whatever he threw, we took off him and put it out of reach but where he could see and when he asked for it, we'd say no and remind him why it was there.

It took ages, but eventually he stopped and only does it occasionally now.

I found it very frustrating and so many things ended up getting broken. Fortunately it is a just another delightful phase they can go through!