Either catching him in the act* and telling him he's naughty for doing so ("NO!" "BAD!" What ever works in your house for "You're not allowed to be doing that ever.") If he's an obedient dog, you should be able to break the habit with in about two weeks of consistency. If he's an obedient dog and doesn't quit then he's more than likely missing something from his diet. (I don't care whether you feed Ol Roy, or the best selection of raw free range organic meats and supplement blend... it's a sign that something's missing... EVEN if all your other dogs have done well on the diet you've been feeding them.)

*It would actually be better to catch him as he's ABOUT to chomp down on it. If he just walks up to a pile and chomps, then do it as he approaches the pile. If he's one to sniff it before setting in for a fine meal, than wait until he gets a couple sniffs in... you know... since it's dogs' way of getting the daily news. (Training is mostly about the timing.) If he already gets a mouthful then he's already rewarded himself, and all you are doing is teaching him that you won't let him have it... and he'll get sneakier.)

Now if he's not an obedient dog and you were consistent on monitoring him when he's out and about... that's another matter. Then I'd just stick to other methods suggested... such as being vigilant about scooping or only taking him out on leash. Yes, I know... a complete pain in the butt solution.