I'm 20. I'm in Mensa, I'm at university, I have a decent part-time job, and I'm generally a well-liked, affable person. My brother is 18, he's in sixth form hoping to go to university, he's also a well-liked and affable person, and also has a part-time job (though not as regular as my 12-hours-per-week job, because of school).
Take a random person living in a complete dive near my hometown, a place I was lucky enough to get away from. The so-called 'Lost City', in the West Midlands of the UK. Known as a rough area in places as far as Newquay, Cornwall. Living on benefits more than likely, generally scum, acting like the worst characters from 'Shameless' but even worse than that. I've been in contact with such people thanks to living near them for half my life, and they're not very pleasant people.
Now, I was spanked as a child. My brother was too. The people living on the Lost City weren't, their parents didn't bother with discipline. Now, I'm not suggesting that spanking your child will make them a better person necessarily, but discipline does, and in my house spanking was the disciplinary measure of choice. Both of my parents were raised in that same way too, and they're both well-rounded, well-off, successful people who've had decent lives.
At the end of the day, if it means your kid learns that doing something wrong is bad, then sure, go ahead and spank them. It worked for me, I used to be a bit of a terror when I was little, as did my brother, but after my parents started spanking us when we were naughty we soon learned to behave. And we're all the better for it. So speaking from personal experience, I can safely say that in the right circumstances, then yes, spanking is acceptable, and indeed should be promoted as a method of discipline. Things like ASBOs and the 'naughty step' don't work, so why not a sharp shock to bring kids back into behaving themselves?
Oh, and you can quote me on this, I also believe that for the same reasons, the cane should be brought back in schools. Bring back corporal punishment, and allow teachers to use whatever measures they feel are necessary (with a few regulations for common sense, but otherwise whatever the teacher deems appropriate) to deal with unruly students. And maybe consider bringing back National Service too. Showing young people discipline and literally forcing them to adhere to it would mean a much better society overall, I think.