You said you are not interested in losing pounds. Do you not have a layer of fat covering your middle or have I misinterpreted your message?
I do know that spot exercising will get you nowhere if that area is covered in fat. If you keep overeating as you explained you will keep adding more.
Yes, you are right. Diet is the answer since you admit you are a lucky "thinny"

and you are already doing cardio and weights.
I eat 6 meals a day. 3 of those are PVL protein/carb drinks. I started 9 days ago and have lost 3 pounds. I'm not hungry. I exercise. I still haven't seen my svelte abs, but I will once I get rid of my excess baggage.
I use a stability ball for ab work. Check out a video on "how to" and give that a whirl. You need to work all areas of your abdomen and the ball is a good way to do it.
Maybe someone else reading our messages has some other answers. I sure would like to hear them myself!