This kind of reversed iconoclasm - even if the flag does represent an objective evil - is not useful in converting a group of people that views itself to be on the side of compassion. The action would make the other side feel validated in their ways.
I would turn to the pro-life movement if you want effective example of enlightening a population. The focus on the pro-life movement is more-so on the value of life and its charismatic appeal to that part of the human heart & conscience which has not been darkened. It is a force of attraction. And you know what? It seems to be working. This isn’t to say you have a vow of silence over the evils of abortion, but when stating everything that is bad becomes the focal point of your ministry, you lose your attraction. It is interesting to note that Nietzsche, Marx, and Freud were all very vocal in accusing mankind of many evils, and yet St JPII points out how lacking they were in offering any solution.
Now that the Supreme Court has yet again opened the door to another evil, I think as Catholics we will need to look to our successes in the pro-life movement as the model to nourishing the sanctity of marriage as well. That’s the focal point: sacred, beautiful, blessed marriage & family.