OMG I am so glad that it is not just Spencer that does this. Some days he gets up and then spends all day throwing everything and anything. I also don't know what to do. Last week we had a particularly bad day where he was getting out of time out (something he had never done before) picking up an object and throwing when he was already in time out for throwing
I have taken to confiscating as well as putting in timeout and depending on what and how he throws depends on whether he gets a warning or not. I.e Block thrown with force = no warning, teddy thrown half assed = warning first. So far he hasn't really cared about me taking stuff off him, but today I bought him some new cars that he seems very attached to so it will be interesting to see what reaction it gets if those get confiscated.
Sometimes, and this really irks me, he will be in the launch position waiting for me to look at him before he throws and most times not even waiting for me to look he waits for me to say "don't throw" and then he throws it and looks at me as if to say "what you gonna do about it?"
Over the weekend we got him some balloons and that I think has helped a bit, we did have a couple of balls that we allowed him to throw cos they don't do too much damage, or they didn't until he learnt to throw them at the kitchen worktop or straight at the tv etc.
So I dunno what the answer is, will let you know how we go with the confiscating. Oh and I'm also hoping the weather that perhaps we can do the whole we don't throw inside thing, but then maybe not as launching stones at the car has become a new favourite

and I don't want him to think it is ok to lobb stuff at playgroup just cos we are outside.