I wrote a HN post about it as well: https://news.ycombinator.com/item?id=26105890, but to spare you all the irrelevant details and digging in the comments for updates - here is what worked for me - you can block all their IPs, even though they may have A LOT and can change them on each call:
1) I prepared a fake URL that no legitimate user will ever visit (like website_proxying_mine.com/search?search=proxy_mirroring_hacker_tag)
2) I loaded that URL like 30 thousand times
3) from my logs, I extracted all IPs that searched for "proxy_mirroring_hacker_tag" (which, from memory, was something like 4 or 5k unique IPs)
4) I blocked all of them
After doing the above, the offending domains were showing errors for 2-3 days and then they switched to something else and left me alone.
I still go back and check them every few months or so ...
P.S. My advice is to remove their URL from your post here. This will not help with search engines picking up their domain and ranking it with your content ...