I don't think spanking is especially wrong, per se, I just think it's likely to send the wrong message. When you spank a kid, you're not putting into their mind that what they did is wrong, you're putting into their mind that what they did results in pain.
Maybe some people might view them as one and the same, but they really aren't, and I believe making that distinction makes all the difference. Firm boundaries, rules and, in the event of misbehavior, discussion and punishment within reason. Talk to your kid, make them understand, and then, take away the TV or the video games for a few days. That's what I believe in. That's how I was raised and I turned out okay, after all.