Hope things are still going ok. As you have said yourself, the whole "don't smack" thing is ridiculous. It's naughty to smack so I'm going to smack you!
I think understanding why the child is behaving the way they are is key to addressing the behaviour. And it is the behaviour you need to address as much as the way you deal with it.
I bet your older child is jealous, plain and simple.
Of course, if that's the case - then every single time you smack him, you reinforce his feeling that you love the baby more. So he'll take it out on the interloper even more.
Distraction. Praise for being gentle with the baby. Some 1:1 time with him. Lots of telling him you love him. And just moving him away with a firm "no". And when he hits his brother, you sweep his brother up and give him the attention. When he is nice and gentle with his brother, you sweep him up and give him the attention.
He learns that he best gets mummy's attention by being a nice, gentle boy.
There's 15 months between my two, and the first thing my eldest did when he met his little brother was poke him in the eye 
I will never forget one instance. This was during a period when my eldest was horrible to my youngest. My youngest was just sitting silently on the sofa, with tears rolling down his face. Not even making a sound. Just total misery, beyond being able to sob.
From that day, he got bucket loads of attention and whisked into my arms every time his brother bashed him (and I hadn't been able to head his brother off or distract him beforehand) It meant whisking him into the air as a blow was on its way! So you have to be there, all the time, ready to intervene, until you have trained them to know that the best way that they get attention is to be gentle. That lashing out has the opposite effect to the one they were going for.
It's a long process. And siblings will always fight! You'll never get rid of that. But you can get rid of the bullying.