Hi! I would definately get ontop of this one straight away as the longer you let him laugh in your face when he's done something you don't like, the worse it's going to get & pretty soon you'll have no control whatsoever.
What worked for us was the "Naughty corner". I don't know if you've seen the super nanny, but this is her method for discipline. Firstly find a good "naughty corner", somewhere you can still keep an eye on him but where there's no stimulants, you can also put a chair there or a mat to designate the spot (naughty chair, naughty mat etc). When he's misbehaving, give a warning & do as you have been and get down to his level with eye contact and tell him in a stern voice "(insert name), you DO NOT throw toys. That behaviour is unacceptable & if you do that again you're going to the naughty corner". Then get up and walk away.
If he does it again, place him in the naughty corner for 1 minute of every year he is (2 minutes for 2yrs old etc), and tell him why he's there"(insert name), you're in the naughty corner because mummy told you stop throwing toys and you continued to do it. Now you stay in the naughty corner for 2 minutes" & walk away.
If he keeps leaving the corner, chair etc .. tell him the first time "you need to stay in the naughty corner for 2 minutes!". If he continues, keep placing him back without speaking as then he's getting a reaction from you (you must remain calm & stern at all times).
After the 2 minutes, come down to his level again & explain to him again why he's there and that it is unacceptable behaviour. Then you must get him to say sorry to you "Now you need to say sorry to mummy", and don't let him out of the corner until he does. If he refuses, tell him it's another 2 minutes in the corner until he apologises. After it's all over, go back to being cheery and positive. You'll be surprised at how quickly this can work if done correctly, we've been doing it for about a year now and it works very well for us.
If you wanted to add another element to it, every time he goes to the naughty corner, you can take 1 toy away and put it in the "naughty box", and he only get's it back when he does something good. Can be anything from picking up some toys, doing something when you ask him to etc etc. And don't be scared to keep doing it until all the toys are gone, he'll get the idea pretty quickly!
It's worth a try anyway .... Good Luck & let me know how you go if you decide to try it.
Jaida (6), Ameli (2) & Lacey (2mths) - SA