Simply ask them if they can please check their partisan politics at the door. It will put them on notice. Then write a letter to the editor of you local paper discussing the issue. Attend and bring it up at a PTA meeting. If you want to sink that boat you have to start making waves.
PTA is a political tool of the NEA. By joining your local PTA, you are automatically sending $2.00 to the National PTA which supports all the same things as the NEA. We refuse to join, though we remain active in the school.