Just so they know that you got their message loud and clear. And you are answering them loud and clear
You know it would make her blood boil to see that. It is likely a female because of the way she worded the message. She is likely next door or across the street because she feels FORCED to see your offending "HILL BILLY" car sticker and it grates on her daily. So much so that she felt the need to retaliate. She has also made a judgement of you being a "hill billy" which is another clue she's a very close neighbor. So you've got it narrowed down to like 5 neighbors at most. Now, because she's a woman of anonymous action, she will not be able to help but go complain to someone about your new HUGE sign. She may even come to complain to you. But she will probably only drop you another chicken heart-ed letter.
Might even want to set up a hidden time lapse camera if you really want to talk to this woman and try to work it out. There is a good camera called "PlantCam" on Amazon that is weather proof and you can set it to take pictures in various time frames depending on your need. You could at least pin point her on camera ant then confront her as the reasonable constitutionalists that you are and try to educate her on how you actually make her children safer considering the response times of police. IF that doesn't work, you will at least prevent her from doing anymore anonymous stuff destructive or otherwise.