maybe its your own god damned fault for putting yourself in a position where they had a copy of the finalized album AND the album cover before they payed you, extremely bad practice. Unfortunately Loren already posted because I highly believe that it should be a lesson learned for doing business in a way that allowed you to be a victim. Almost once a month there is always a thread here about someone whining and bitching because some band ripped them off after the engineer handed them a copy of the recordings before they paid.
You asked for it dude. Rather trying to get back at them in spite, just learn the lesson and if they ever come up as a topic in the local area, professionally tell people about their extremely unprofessional way of doing business including stealing from producers that they hired. When venues hear about that they won't want to book them out of fear of being victimized by the band.