There isn't just one definition of "violence" and the word is sometimes defined to include a malicious intent. Rarely would one say that a surgeon violently injures his patient when performing an operation or that the first responders administered violent CPR. Usually people project their own moral value judgements onto the word so that only what they deem as bad or evil acts are "violent". This is less a matter of fact than it is a matter of definition.

As for whether spanking is the most effective option, the jury is still out. It wouldn't be surprising to find a correlation between spanking and slightly lower IQ or later behavioral problems, but does spanking cause either? Can scientific literature that relies on self-reporting by parents show anything useful?

Regardless of effectiveness, should one spank? Is it morally wrong? What basis do we decide the morality of it?