As far as I know, there isn't really a good system for dealing with consumer complaints in France - no "better business bureau" at any rate.

If your French is up to it, you may want to consult the magazine 60 Millions de Consommateurs Home - Le site du magazine 60 millions de Consommateurs or just pick up a copy at your local news stand. They are supposed to be able to help out individuals who have problems with products and tradesmen, or at the least, to be able to recommend the appropriate agency or resource to go to.

If you really can't work it out with the plumber, ask at your local mairie to see where and when they hold the "meet with a notaire" sessions in your area. You can get a 15 or 20 minute free consultation with a notaire where you can explain what happened and determine if you have any legal course of action.

Don't reveal to the mairie the name of the plumber you're having problems with (all France is "small town" and everyone is related to everyone else...) - but they may also be able to point you to some sort of consumer association as well.

These are just "off the top of my head" suggestions. I'm outside France at the moment so can't ask around like I normally do. Will follow up when I get back home later this week.
Cheers,
Bev