Aidyn went through a hitting stage (as well as throwing) between 1.5yrs and 2.5yrs old. We just had to firmly repeat 'no hitting) (etc) over and over again. And simply explain that it is not nice to hit, as hitting hurts people. It sunk in eventually.
Although I have seen kids occasionally doing it at daycare, I don't think that is where he picked it up, I think it just came naturally to him out of frustration.
Now he has moved onto pinching us occasionally - purely for our reactions I think, so I am trying to nip that one in the bud too.
Good luck!