Positive reinforcement wasn't enough for Hunter when getting him to stop barking at dogs through the fence. I blocked off the view and gave him a very strong 'NO' whenever he started, he picked up my tone and knew I wasn't happy with him so he'd stop. He's pretty quiet now.
Also, other dogs barking seemed to set him off. So I'd sometimes play barking audios on youtube during training to get him to stay calm when he heard barks; he pretty much ignores dogs barking now.
Obviously every dog requires a different training method but I think the most important thing is to find the cause of his barking and work with that.