I know there are dog people on this board, so I'm hoping for some advice!
We adopted a wonderful foxhound a year and a half ago. Luckily for us, we have had only very minor issues with him which have all tended to revolve around him not getting enough exercise. So long as he is well-run, he is absolutely no problem at all and is a joy to be around.
Until now.
We were recently away for 10 days, and he stayed with my cousin. Ever since we've been back, our dog has been a real PITA - mostly by stealing food. He takes food from the counter. He takes food from temporarily unmanned plates. And it's driving me insane.
I came home today to find an (expensive!) jar of honey half eaten, which had been spilled all over the carpet. Yesterday, the peanut butter jar was broken into and totally consumed - and peanut-butter paw prints on my wood floors. Plus, it makes me sick to think about those paws on my table and countertops! 
We really do clean up well after ourselves and don't leave open food around. So I'm at my wit's end! We NEVER had this problem before the damn dog went to my cousin's.
So how does one un-train a previously well-trained dog? I've got two kids who I'm constantly cleaning up after, and really don't have the time or energy to deal with this.
Help!
You say that he has been a PITA "mostly by food stealing". What else has he been doing? I only ask because sometimes stealing food can be a manifestation of separation anxiety.
It seems from your description that he is stealing food both in your presence and while you are gone. Stealing food is unfortunately a difficult behavior to stop as it is food and therefore a natural, normal behavior that is self-rewarding.
If your dog is going after food in your presence, and you would like to retrain him there are a few things you could try: Start by feeding him before the humans eat - this is a simple step that might help him be more satisfied and less likely to steal food from plates. You can also try feeding him two or three smaller meals instead of one big one (if that is what you are doing). For retraining purposes, you could start by keeping him by you on a leash with a head halter (the Gentle Leader is great for this) and discourage him using remote punishment.
Now, punishment is not something that is often recommended by veterinary behaviorists as its very difficult to use effectively and can often create anxiety or aggression problems. But! It can be useful when applied correctly.
The main principles involved in effective punishment include 1. Immediacy. Dogs cannot be punished effectively if more than 2 seconds have passed between the activity and the punishment. They simply cannot tie the two events together and whatever punishment has been meted out after that will be ineffective and/or inhumane. 2. Consistency. They need to be punished *every time* or as close to it as possible (which is why I suggest keeping him attached to you by a leash) 3. Appropriateness/intensity of punishment. This is where the "remote" punishment comes into play - often a squirt of water or a sharp, aversive sound (like a short "ah ah!") work well. Yelling or hitting are really not appropriate - these are the kinds of things that can also lead to anxiety/aggression problems!
If he is stealing food mostly when you are not around, you might have to end up housing him in a situation where he can't get at food. Maybe an x-pen or baby-gated away from the kitchen?
Another idea might be to put some of his food in food-puzzles or kongs where he has to work at getting his food, which you can leave behind when you go. This might distract him enough where he might stop trying to get at the "forbidden" food.
Another tactic might be to train him to go to a spot on command when people are eating or food is around. PM me if you want more details on how to do this - I fear I am probably already going on and on a bit!
ETA: You didn't mention how old your dog is, and I forgot to mention that, as is often the case with behavioral problems - there are also a lot of medical reasons why he might be obsessed with food or polyphagic. So, I would also suggest a checkup with your vet if this is not easily resolved. :-)
Best of luck! Feel free to PM me I love animals and behavior so I find these kinds of cases fun!