Get a soft muzzle. I know alot of people don't like them, but they work when used correctly. As soo as the dog starts barking you put the muzzle on (they cann't bark with it on) saying 'quiet' in a firm deep voice. After a half hour you take it off again. As soon as the dog starts barking again, you put it on again. You repeat this process for as long as it takes, be prepared to start this on a day when you can dedicate alot of time towards ptting it on and off. eventually just the sight of the muzzle will stop the barking and then from there you won't even need the muzzle anymore. The dog may still occaisionally bark, but should quiet as soon as you say the word quiet.

My lab was easily trained not to bark with food, but my beagle needed the muzzle to help train her. She hates it, and I hated using it - but it only really took a couple of days and then once or twice when she regressed a bit a month down the track. But now, I am not even sure where the muzzle is. She will still bark to warn of people on her boundary - but it is a quick bark (a warning) and then goes silent, unless the person actually crosses the booundary without myself or DH present.