My neighbors are stealing my water.?

They just hooked their hose up to my water spigot and filled their pool when I'm not home, TWICE! I had my landlord pull their water bills and mine for the last several months. So far they have stolen $300 worth of water from me in one month. He says that he knows they are doing it. The water company says there is no doubt that they are doing it, but yet, it seems like I am unable to do anything to recover the $300. Any suggestions?
A hidden video camera and small claims court.

Also, call around to hardware stores and see if there is some sort of locking mechanism that you can put on your outdoor faucets. There must be something, sort of like the things you can use for your heat and AC to keep people from adjusting the temperature.
Call the police. That's theft. If they came to your house and stole food out of your fridge it would be the same thing. You have witnesses and documentation and a 'filled pool' that proves your allegations so present all your info to the police so they can make a full report. If they don't pay you back take them to small claims.
What a bunch of idiots. Thats one of the funniest things I've heard though but bring them to small claims court and get your money back.
It is rather obvious that you don't live in New Jersey. We know how to handle situations like this. With all of the electronic surveilance systems on the market I am sure you could hook something up to catch these people. Once you have proof of theft then you have them in a very compromising situation. From that point on you own them!
It sounds as if your best option is to file a complaint against them in civil court or small claims court as a minimum. If you don't have the money for an attorney, then small claims court will be your best option.

Hopefully, your landlord and someone from the water company are willing to testify on your behalf. In the meantime keep getting all the documentation you can.

Other options would be to pretend that your not home, wait until your neighbor comes over and catch him in the act. Walmart also sells faux surveillance cameras, you can 'install' one above the spigot and hope your neighbor sees the cameral and changes his mind. Perhaps your local home and garden center has a device you can put on your spigot to prevent unauthoized use. The last and most obvious option is to confront your neighbor, say look I know you've been using my water and I have evidence to prove and if you do it again I'll sue you, and then walk away.
You can take them to court for theft. You bring along any and all evidence in addition to bringing along your landlord and also a representative from the water company. (If the landlord and representative from the water company cannot come with you on your day in court, get a notarized statement from both parties or perhaps you can subpeona them to appear.)

Take them to small claims court and present the bills and records to the court. And, put an inside valve on the line to the outside spigot.
Sue them in small claim courts; you can also press criminal charges for trespassing and stealing water if you catch them in the act.

Another option it would be to remove all outside faucets. Locking them might not be useful if he knows how to pick the locks. The video surveillance might be good if you have the time and patience to do it.
I would start by sending a registered letter outlining your complaint and a demand for payment of the water bill. If they don't pay, file criminal charges! These people are thieves and should be punished.
Charge them with theft and take them to small claims court. Put a valve on the water pipe on the inside of the outside spigot and just open that valve when you need it.