I really wish I had some good advice, but all I can say is that Aidyn has gone through that phase recently too, but now it seems he has moved onto throwing things around (and I mean anything, lol)

I havent seen him hit other kids, but he has hit DP and myself out of frustration. Or he will hit himself or a nearby piece of furniture. This especially seems to happen when we tell him not to do something (ie. pulling the decorations off the christmas tree)

When he hits us though I sternly say to him 'don't hit mummy/daddy Aidyn' and sometimes will add something onto the end such as 'hitting hurts' or 'I know you are feeling frustrated but hitting wont solve the problem' (not that a 18 months old would understand that though!)
But anyway, once he has been told off he comes and gives us a cuddle... which is really sweet, as if he's saying sorry. Although on the other had I suspect it could be a bit manipulative too?

ARGH... anyway, I have given you absolutely no constructive advice here, sorry!
I would advise to just keep saying 'no' in a firm voice, and hopefully he will soon realise he shouldnt be doing it, and will get over the novelty of it?
I have found that Aidyn is now at the age where he is very aware of what 'no' means, and although it frustrates him, he usually does learn to obey they rules after a time...