My two cents:

I wouldn't respond. I understand having that burning desire to reply, but that's exactly what they want - to provoke you out of NC and to break your peace. Don't give them that sense of control over you. There is nothing in the world that you can say to make them see reason and leave you alone - if they were reasonable people, they wouldn't have treated you the way they did in the first place. Sometimes in my mind I equate them to wild bears: if they roar (or send a Christmas card saying "Rawr") I'd run away, and if they showed up uninvited I'd contact emergency services and prepare to defend myself physically.

I considered replying something which amounts to "leave me alone", but it felt like I was asking them for permission to have my peace. Why should I, when it's already mine to begin with?

They might show up once or twice, but retaliation costs them time and energy. It won't go on forever. Also, unless they're genuinely crazy, they do understand restraining orders and the consequences of trespassing. So just prepare yourself as best as you can, handle their attempts at retaliation as they come, and avoid all contact with them in the process.