As much as i'm not into physical reprimands as punishment.. i must add one comment to the earlier one made..

If you slap the childs hand and it doesn't make them 'start', then it was like playing paddy-cake and has no effect.. I am not saying smack em hard or leave a bruise. Just don't tap their hand and think they'll understand..
Now tapping their hand and in a loud firm voice saying "NO" might work better.. But mean it when you say it!! If i hear you say that to your kid and i don't look to see if a backhand is gonna follow, then it wasn't said firmly enough!!!