In this case i disagree with "The bad guy". Don't take action against them, just let them be. I recommend to ignore them.
Please look at this on a logical way. If you take personal offense, then you take ownership for those insults. If you take action against them, if you really act based on the fact that you feel offended because they insulted a group in a general way, then you are just a brony, no better, no worse than any of us. That includes the guys who call themselves bronies and fuck horses. If that facebook page gets to you, then you are one with the best, but also the worst of bronydom. You will become whatever the bronies become.
On the other side, if you just let them be, if you do not let them to hurt you, then they have no power. Be bigger, better, stronger, and they are just a bunch of ignorant bitches whining on facebook. This will not be a problem anymore, it will become a challenge that you automatically have won. If you can look at that and say "That's not me" you will trascend being just part of the group, and will be the one who sets trends, the one who people follow. You will not be Just a brony, you will be what Bronies will become.
So, what are you going to do? Are you going to earn and own those insults or are you going to be the bigger one?