Good old honeytrap. I'm not sure, but I think that it's doable.
Have a honeytrap page somewhere in your website. Make sure that legit users won't access it. Disallow crawling the honeytrap page through robots.txt.
Then if some crawler still accesses it, you could record+ban it as you said... or you could be even nastier and let it do so. Fill the honeytrap page with poison - nonsensical text that would look like something that humans would write.
I think I used to do something similar with email spam traps. Not sure if it's still around but basically you could help build NaCL lists by posting an email address on your website somewhere that was visible in the source code but not visible to normal users, like in a div that was way on the left side of the screen.
Anyway, spammers that do regular expression searches for email addresses would email it and get their IPs added to naughty lists.
Yup, it's the same approach as email spam traps. Except the naughty list, but... holy fuck a shareable bot IP list is an amazing addition, it would increase the damage to those web crawling businesses.
but with all of the cloud resources now, you can switch through IP addresses without any trouble. hell, you could just browse by IP6 and not even worry with how cheap those are!
Yeah, that throws a monkey wrench into the idea. That's a shame, because "either respect robots.txt or you're denied access to a lot of websites!" is appealing.
For banning: I'm not sure but I don't think so. It seems to me that prefetching behaviour is dictated by a page linking another, to avoid any issue all that the site owner needs to do is to not prefetch links for the honeytrap.
For poisoning: I'm fairly certain that it doesn't. At most you'd prefetch a page full of rubbish.
Yeah, this is a pretty classic honeypot method. Basically make something available but inaccessible to the normal user. Then you know anyone who accesses it is not a normal user.
I’ve even seen this done with Steam achievements before; There was a hidden game achievement which was only available via hacking. So anyone who used hacks immediately outed themselves with a rare achievement that was visible on their profile.
There are tools that just flag you as having gotten an achievement on Steam, you don't even have to have the game open to do it. I'd hardly call that 'hacking'.
Better yet, point the crawler to a massive text file of almost but not quite grammatically correct garbage to poison the model. Something it will recognize as language and internalize, but severely degrade the quality of its output.
You're second point is a good one, but you absolutely can log the IP which requested robots.txt. That's just a standard part of any http server ever, no JavaScript needed.
People not intending to follow it is the real reason not to bother, but it's trivial to track who downloaded the file and then hit something they were asked not to.
Like, 10 minutes work to do right. You don't need js to do it at all.