Some "less technical" suggestions after being inspired by other creative suggestions here:
Put brandings/personalizations/signatures in your pages that are not easy to remove to remove automatically. Include your site URL if possible. The idea is that if a visitor sees these on a different site, it becomes obvious that the content doesn't belong there.
Write an article page about these things happening, specifically mentioning the mirroring site URLs, and see if they will also blindly mirror it.
Put brandings/personalizations/signatures in your pages that are not easy to remove to remove automatically. Include your site URL if possible. The idea is that if a visitor sees these on a different site, it becomes obvious that the content doesn't belong there.
Write an article page about these things happening, specifically mentioning the mirroring site URLs, and see if they will also blindly mirror it.