You can place another bagau in your window and your neighbor will place a pair of Fu Dogs and then you will place a coins sword to fight back. hahahaha.. Just kidding
Actually in Feng Shui a Bagua mirror is of no significance and is just being introduced by present modern practitioner. According to them it can reflect off poison arrows. In ancient classic Feng Shui, bagau mirror was made of brass and was to introduce some metal element into an afflicted sector. Bagua mirrors are used for spiritual purposes and to ward off spirits in certain circumstances. But in Feng Shui Bagua diagram indicates mathematical model of the universe which is based on the polarities of the Yin and Yang.
It is just in our minds of those who believe in them but if you feel eyesore. then I will suggest that you just close your window curtain.