Here's one suggestion:
Call them up and get the name of the tech that did the work, the service manager and the owner or store manager. Tell them you need these names in order to file a fraud claim with the attorney general (or whatever law enforcement entity you choose).
It seems to be that by doing work that is unnecessary, then admitting that it was unnecessary, and refusing to refund all costs for the unnecessary part of the work (including labor), they have admitted to fraud.
I could be mistaken, but if the request for names doesn't shake them up, you could then give the info you have to law enforcement and see if that will shake their tree....
If you paid by credit card, you could also see if you can get some assistance from the credit card company....