I've never smacked my cat, because I've never had to - if she does anything untoward she'll run off rather than face the music. However, I have given my dog a bit of a tap on occasion. All the dog training manuals etc. say never to do it, but there are times when she won't listen and it's the only way. Nothing painful - for instance a tap on the nose if she tries to put it on the table while I'm eating - but just enough to get her attention. However, if she or the cat were hanging on to me with teeth or claws then I can't say I wouldn't use the quickest and most effective way of making them let go.