Go in personally and threaten to bring them to small claims court, if that doesn't work tell them you are devoting the next year to bad mouthing their store in every way possible.
In most cases, threatening legal action is a bad idea. It shuts down any further dialogue, and most businesses know it to be a bluff. If you are
going to pursue legal action, don't mention it to them. Let them find out when they get the summons.
As far as word-of-mouth...some stores take it very seriously, while others disregard it completely. A store that's going to rip people off is probably going to fall in the latter group.
I'd take it to the local magistrate court.