Every time I see "parents" like this mentioning spanking, I see people from all sides defending it as "having its uses". The thing is, not only is it inconsistent with children's rights but it's also ineffective. The use of violence to teach children /anything/, is consistently linked in developmental psych research with increased disciplinary problems and a child's tendency to use violence as a problem solver. Furthermore, research on what encourages people to use spanking as a disciplinary method has always turned up a TON of responses along the line of "Well, that's just how you teach children not to do really bad things when they're too young to understand..." often without defining when they are too old to understand. Others more blatantly say "It was done to me and I turned out fine", which is flatly contradicted by the research in the majority of cases.
The problem is, as long as our society incorporates elements of permissiveness without setting firm boundaries, an individual will be free to overstep the rights of their children and abuse them under the disguise of "discipline". Separating "spanking" from "abuse" is similar to the imaginary line we have which separates "bullying" from "criminal assault and harassment". When it happens to children, it's okay. But doing it to an adult would be wrong, of course. Because adults are better? Because children can't be taught using words and example? None of the excuses bear out under the research.
Spanking is not necessary, not even as a last resort. It's consistently been shown in the aforementioned developmental research that authoritative (not authoritarian) and nonviolent parenting, in countries where all forms of corporal punishment are flat-out illegal, produces children who know the difference between right and wrong, and are no more prone to dangerous behavior than any other children.
If your child is too young to understand why they should not perform a dangerous action when you explain it with words, the solution is not to beat them until they are scared of performing the action. Pain as a motivator for behavior encourages pain as a problem solver by showing children that the way to make people do what you think they should be doing (for good or for ill) is to hurt them. It's been shown that consistent spanking (not abuse, just spanking as a punishment for wrongs committed by children) has a strong link to adult spousal abuse, criminal problems, and violence in general. The research literature can mostly be found in the journal of american psych, but if you want to get some really in depth information on why spanking children is a flat-out bad idea, it may be worth checking out the work that's come out of the United Nations Council on the Convention for Children's Rights.
Further work has recently been compiled in books by writers like Howe and Covell. For one book which points to and summarizes much research, Howe and Covell's "Children Violence and Families" which was published earlier this year does a great job of visiting the spanking issue.
Just felt it was useful to mention because I saw lots of people defending spanking.