Stop Dog from Barking -- Collars

Ultrasonic collars can be used to stop barking dogs. There is some debate about whether ultrasonic collars are humane. The trouble is we humans can't hear ultrasonic sound so it's hard to make a call on whether this is too loud or even painful for a dog. However, they all work on the idea that a loud noise that will startle a dog and cut off the barking.

Humans can't hear this but dogs can as it's an annoying, high-pitched sound that is triggered whenever the dog barks.
For collars the sound is set off by the dog wearing the collar; however there have been cases of these being set off by other dogs' barking, or even other loud noises.

For dogs that don't belong to you, there are devices that generate an ultrasonic noise if there is a dog barking in range. A popular one is "Bark Free" although they all work on the same principle most have a range of between 20-70 feet. They are mounted outside as close as possible and ideally in line of sight of the barking dog. When the dog barks, the device emits a tone that acts as a distraction that startles the dog to stop it from barking.

Sonic collars operate on the same principle as ultrasonics, but this sound can be heard by humans. You have to hope the dog stops barking or you'll have 2 noise sources.

Citronella Dog Collar
The citronella-spray collar works on the principle that a mild unpleasant smell can stop barking. Most of us are familiar with citronella from anti-mosquito candles. Citronella is a natural oil with a lemony smell that is quite pleasant for us but unpleasant for dogs (but not inhumane). I am not sure I would fancy a spray of Citronella up close. Dogs eventually figure it out...although some revert to barking when it is taken off. To be effective any measure has to be consistent and removing the collar is not consistent. These can also be set off by other dogs...a tad unfair and confusing methinks. It seems a reasonable step to try if you are running out of ideas and training hasn't worked.

Pinch Collars
So you are at the end of your tether. You have what looks to be an un-trainable dog. A more serious measure that has been around for a long time is the pinch-collar. These collars imitate what their mothers used to do to correct them as puppies...a sharp nip on the neck.
Most trainers recommend having the dog wear it all the time so that when the dog barks and she doesn't stop immediately, give the collar a tug. This effectively stops the dog in mid bark most times. And follow-up with praise and a pet. The pinch collar interrupts the unwanted behavior. Big drawback is that you need the lead to be on and you to be right with the dog.
If you decide to do this, get properly trained in its use and how to put it on your dog. And never never never use it as a punishment. This is a questionable technique for me as it is definitely painful to the dog and if you get angry it may be easy to pinch too long.

Choke Collars
Choke collars work on this same interruption idea but with no link back to being a puppy. They compress the dog's windpipe and for that reason I personally don't like them.

Shock Collars
Shock collars arte plain and simple punishment, they give an electric shock whenever the dog barks. Some models you can adjust the voltage. They will stop barking eventually but don't get to the cause of the barking and the shock is unlikely to be understood by the dog. Some trainers think these cause more aggressive behavior as the dog gets frustrated angry and scared. I would hesitate before using these.