My friends and family often approach me and ask for guidance on how to resolve issues with their General Contractors or trades. While each issue is very different, one common recommendation I give to everyone is to be firm but respectful. It's easy to let your frustration and anger get the better of you but in the end, it only makes things worse. Even if you know the contractor is wrong, you need to keep your cool and keep the dialog open so a resolution can be found. If the contractor shuts you out, I guarantee the issue won't get resolved and you'll be a lot more angry than you were.
Another common problem I get asked about is what to do when the contractor springs additional charges/costs on your job. Unfortunately, overage and extra costs are almost always part of deal with renovations and it is very hard for most customers to know what is real and what is inflated since these aren't things you purchase on a regular basis. My best recommendation is to ask your contractor for a detailed description and breakdown of the additional charges (labor costs, materials, equipment, etc) and to ask for copies of the receipts for the materials (if any) that contributed to the extra charges. If they have nothing to hide, the contractor will gladly comply.
If after all of this, you are still not able to get resolution to your problem, you should consult with an attorney but be warned, this is not cheap so the price tag in question needs to be high to justify this approach. You also have less drastic but effective approaches like filing a complaint with the Better Business Bureau and/or leave negative reviews online. Both of these often end up re-engaging the contractor to help avoid any negative ramifications to their reputation and future business impact.