Unfortunately, there is no way to limit the use of the name "Montessori." Parents must carefully research, and observe a classroom in operation, in order to choose a real Montessori school for their child.
There are several Montessori organizations to which schools can belong. The two major ones operating in the United States are the Association Montessori Internationale (AMI, with a U.S. branch office called AMI-USA) and the American Montessori Society (AMS). Parents considering placing a child in a Montessori school should ask about the school’s affiliation(s).