D.P.
My 21 mo old started slapping, headbutting, and pushing at around that age. We use words like "Gentle" and "Nice hands" and try to show by example, but he is VERY willful and it just doesn't stick sometimes.
We also used to put him in his high chair in piano room (which is very boring) for a 1-2 minute time out. Now, he is in a booster and the high chair has gone into storage. We still do time out in a different chair, but we have to threaten the "paddle" if he continues to get out of the chair. We only had to pop him once on his little pamper butt with a wooden spatula for the "paddle" concept to stick. Now, he doesn't try to fight anymore and begrudgingly accepts his fate (but not without a HUGE fuss!).
As for the behavior- it has gotten better. He still tries to push kids smaller than him down from time to time. We still have to keep an eye on him. Lately, he has been doing very good about saying he is sorry. So, we are having small successes.
Good luck!