Contact your local propane company, fire department or a company specializing in removal of hazardous materials. They will be able to relay more information concerning local laws.
Hire a local disposal company. Employing unlicensed workers to remove gas may not only be illegal, it can cause serious damage, bodily harm and fire.
Use a licensed disposal company to transfer remaining gas to another container, depressurize the tank, remove all fittings and properly dispose of empty containers.