what happens if they won't be reasoned with?
There are all sorts of punishments you can use. Denial of privileges - no TV, games, after-school sports etc. With little children being made to sit apart from the family for a while 'the naughty step' can be effective.
Even more important is noticing and rewarding good behaviour.
It takes time, patience, and consistency. Act like an intelligent adult.
In the UK it is illegal to smack using any implement or to use your hand in such a way as to leave a mark. In Scotland the legal defence of 'reasonable chastisement' is set to be removed this year making smacking illegal.