I am so sorry to hear of your situation. Yes, life is too short for this harassment. As tempting as it may be to fight fire with fire, in my experience, it only makes matters worse as people like this are literally looking for a fight.
I suggest showering them with love. Take them home baked goodies, etc. And yes, smiling when outside even when you just want to scream. They, honestly, won't know what to do.
I have to admit though at our last house (we were out in the country and the neighbors were some distance from us) we found out from another neighbor that the neighbors directly behind us were spying on us with binoculars (among other strange things). There was only one window they could possibly see through because of trees and shrubs, we decided to be naked and put on amorous 'shows' in front of the window whenever possible. Give them something to talk about! We don't know if it worked, but they did move before we did. We just thought their behavior was sad and funny, we didn't let it bother us. We left them alone to their misery.
One thing I would consider is when they stand outside looking in, walk to the window while naked and pull the shade. It's your house, you can be naked anytime you want! They deserve to see what they see! (The sight of my aging overweight body, may burn their retinas!)
In the long run, selling and moving on may be the best option. Good luck, in whatever you decide!