Does he *have* to go to preschool? I mean, is this a situation where you both work and need the preschool as a childcare? If not, then if I were you, I'd take him out and let him stay home with you. He is obviously not ready for a setting like that. If you are not able to do that, then you might consider hiring a sitter for him (with no other children). I doubt anyone would want their children around him with that type of behavior. Strong-willed is just a catchy name for misbehaved!
Aside from all that, you both need to nip this in the flower (I'd say "bud" but it's past that now!). You say you've done all these things to punish him, but obviously none have had much, if any, impact on him and you need to let him know who is in authority. It is much easier to do it now, than 10 years from now! And believe me, he will feel safer overall knowing you are in control.
Strip his room completely of any toys or anything he might play with. Sit him down and tell him--WITHOUT EMOTION--that the next time he gets "violent" no matter what time of day it is, he will have to go to his room for the remainder of the day (no matter what time it is!) and will only be allowed to come out for 3 minutes to go to the bathroom. Then tell him his bedtime will be 1 hour early the next day.
Now here's the key: You must stick to this and you cannot let him out of his room no matter what. If he cries, let him cry and if it bothers you, tell him the door must be closed while he cries. If he whines, tell him you do not talk to anyone who whines. If he wants to talk to you about it, tell him that you will talk to him about other things, but you will not talk about his punishment for violence/misbehavior.
And I will reiterate once more: do not back down. This is likely why nothing you've tried has worked--either you backed down from a punishment you gave him, or you didn't stick with it and do it every single time he misbehaved. If you stick to your guns NO MATTER WHAT, he will learn that you are in charge and that he must respect your authority over him as his parents.
Best of luck to you!
A. - Mom of 2 boys, ages 9 & 15 & 1 girl, age 11