When in Doubt...Breathe!

I am the mother of a very stubborn, rambunctious, spirited, and fully lovable four year old named Gavin. When I say spirited, I mean a balls to the wall, 120mph young man who entertains and exasperates me daily. He is also extremely intelligent and has a memory like no other (is it normal for a four year old to recall the color of the shirt you were wearing while shopping at Fred Meyer two years ago?) So as you can imagine, my daily life consists of endless "No! Don't do that's!" and "Can you please just settle down?" as well as "I love you so much sweetie" and "How can you possibly know that? You're only four!"
Yesterday was an especially eventful day. I decided to take Gavin and his five month old brother to go have lunch with my husband (their daddy). So we pulled up to my husbands place of employment, where he is the loss prevention manager (which basically means he watches video monitors all day and tries to catch 70 year old kleptomaniacs stuffing paint rollers down their pants) and started to unload when I received a phone call. Well, my son apparently didn't like the idea of me being on the phone and taking a moment of attention away from him and decided to start using my car as a jungle gym. As I am on the phone I warn him several times..."Sweetie please sit down."... "Gavin, sit down now''..."Sit down now or I am telling your daddy!"..."Alright, sit down now or I I am getting off this phone and spanking your little butt!!"
Apparently Gavin thinks it's funny to see his mommy getting upset, so he decides to laugh at me...can you believe it? A four year old taunting his mother! "Alright THAT'S IT!!" As soon as I get off this phone, you're gettin' it buddy!" So I proceeded to hang up , back my car out of the current space which was very close to the front entrance of the store, and pull into a stall more towards the back of the parking lot where we are less likely to be spotted.
I quickly climbed into the back seat of the car where I very gracefully attempt to pull Gavin onto my lap (all the while screaming bloody murder so loud I am sure the entire state of Washington can hear him) and give him two fairly good whallops on the behind.Now I don't usually like to resort to corporal punishment, but I was at my wits end and felt there was no better way to get the point across to my wonderfully stubborn little boy...besides he was wearing jeans...it didnt seem to faze him...but he did finally realize that mommy was serious. So after invoking my wrath upon him..he decided to settle down and start listening.
We proceeded to walk into the store and meet my wonderful husband outside of his office. After greeting us he pulls us into his room full of video monitors and queues the recording of my little rendezvous with my son in the parking lot.
There it was...mommy in all her glory...face contorted into what looked like a demon... sputtering words of disapproval, disciplining my son when I thought we were out of sight from anyone who might care. Has anyone ever seen what they look like when they've been pushed to the limits and have surrendered to their raw emotions? Not the most flattering of looks I must say.
Won't be doing that again...but what do you do when your darling children just won't listen to you???!!! Do you let it slide or take action and enforce your power at the risk of being spotted by another (hopefully understanding mommy) or your husband watching your every shameful move on video?