Sorry, but I disagree. Piiskata means hitting with some kind of thin object (whip, cat o nine tails, twig, carpet hitting device), whereas spanking means hitting with your hand or with a flat object (paddle).
There is no Finnish direct verb for spanking that I know of, the closest would be 'antaa selkäsauna' or 'läimäyttää (avokämmenellä).
In English, the word spanking is often used when speaking about physical punishment for small children, and in this context especially, it would be wrong to translate that to 'piiskata'.