Sadie,
I'm sorry; What they did is not right.
One option: is billing them & sending the bill with signature requested through the mail. I don't know if a judge would or wouldn't honor this. They might say it is a civil thing & usually you're required to have some type of "contract". I don't know how a verbal contract would work.
2) You could use their own medecine & quote that bible text that says something like YOur Yes should be YES & your no should be no. (Don't remember the exact text)
I don't know if you read my thread on how a JW -housecleaner stole my home made blackberry jam. You can check out the thread. I know exactly how you feel. They have been taught to only think about themselves & that everyone else in the world is evil.
It's not true that JW's are the most honest people. Look how they don't protect the children from sexual/molestation/incest abuse.
I could go on & on ranting, but I'll stop now.