Rather than focusing on whether putting up the signs is illegal or not you should focus on what you would gain out of doing this. Is it out of retaliation? Do you expect positive results?
These problems have been going on for a long time and the issues are getting worse, not better which probably means that it is not being handled properly. It probably never will be and if that is the case you need to be more concerned about how you and your family can best come out of this with the least damage.
It might mean losing money or a sense of justice not being done. These are things that many good people in this world lose out on. What you don't want to happen is that you or your family become so absorbed in this that you become bitter and depressed.
There are times when 'turning the other cheek', although difficult, is the best course. Don't sink to their level.