Appreciate how frustrating poor service can be, but your description of them crosses a line that "could" get you into trouble if they decided to retaliate. I'd suggest you edit your post and remove that last part.
Now for the practical advice, did you pay by credit card? If so, go to your card company and complain and ask them to get involved and initiate a chargeback against the dealer. That will do one of two things, maybe both. First you'll most certainly get the attention of the workshop, second, you may well get your money back
Good luck! Let us know how it goes