Dear Ms. McMasters,
I'm reluctant to endorse to you the notion of spanking as a sensible form of discipline. Many people who were spanked during their growing up years develop perfectly normally; it's not the end of the world to spank. However, it's also not necessary and, in research during the past 10-15 years, we've learned it may produce unintended side effects (e.g., increase in impulsivity and aggression).
My guess is that you would do well by settling on some version of time out, and that your lack of success thus far may have more to do with your implementation than with the merits of the strategy itself. I won't elaborate here on time out tactics, because we've often written about it in other replies (click on the Search function in this Forum for previous notes). Also, you might take a look at the book S.O.S.: Help for Parents (by Lynn Clark) - it is a marvelour 'how to' book re: behavior management and contains, among other things, what I regard as a pretty much foo proof recipe for time out.
One word of advice...you sound like his intelligence may be intimidating you.My son (now 31) was "gifted"( I hate that word).This is what I used to tell him.."My years of experience in this world outweigh anything you have learned at this point from your books.I AM smarter than you and you WILL listen when I speak to you."Try it.