We're being terrorized by a neighbor. Authorites haven't done anything. Can we revoke his greencard?

Unless you're immigration or efen ICE, you personally cannot revoke someone's green card. Anyway your story sounds a little far fetched to me. Why do you have to justify that you want to be a stick and report someone? You'll get support from the haters, who you'll end up meeting in hell, so do what you're gonna do.