My cousin had the same problem and so I vowed only to say NO when it really mattered i.e putting hands near hot oven.
Don't mistake his behaviour as being naughty, there isn't a bad bone in his body! Your son is displaying normal toddler behaviour. If you see him climbing then simply pick him up and say firmly 'can't climb ... or you'll fall'. Once you've let go he's bound to climb again and so just repeat your actions. He'll get bored as he realises he's getting nowhere and move onto something else.
Diversion is another useful method. When he's doing something you don't want such as banging the door, you can say 'oh look at this' and his curiosity will attract him to you.
If you can ignore some of the things he does that annoys you just ignore it. I found my son picking his nose and eating whatever he found inside disgusting. I knew that telling him to stop would make him do it all the more as he was getting a reation from me, so I ignored it. Luckily he has found sweets to be more tasty these days and this habit has stopped of its own accord! It was probably just him exploring new sensations and this is exactly what your little one is doing.
If he smacks another child then move him away and tell him it's naughty to smack others as it hurts them and then go to his 'victim' and lavish attention on them. Your son will not like being ignored.
Unfortunately there is no quick fix and alot of patience is required. It's all about rewarding the good behaviour and ignoring the bad whenever possible. Most importantly is the need for you both to be consistent, it's no good letting him do one thing one day for a 'quiet life' and not the next. Although this stage is tiring it's very enjoyable to watch him make sense of the world around him. I couldn't get angry at my son when one day I discovered him to be shaking talc on my intricate wooden table as he told me he was 'baking a cake'. It was my own fault for leaving the talc within his reach! I have told myself that I should note all these types of things so I can tell him when he's grown up as he's too young to remember. I know it's hard but I find a good sense of humour is required and I can guarantee you'll look back and laugh at his innocent mischief.