If you use Cloudflare to proxy your site, there is a button to click that blocks the AI crawlers (even the free tier). It is almost as if the AI crawlers are a DDoS attack. You can't really do it any other way, since many don't respect robots.txt. At least until someone comes up with crowdsourced blacklists with few false positives.

We have adblockers which rely on open sourced lists of rules. Could we apply something similar to crawlers. Website owners provide a list of IP addresses that accessed them, determine which ones are likely robots and then update the list of websites to block that are likely crawlers. If everyone works together you could probably fingerprint the crawlers as well and block based on the fingerprint. Might increase the cost of crawlers a little won't be fully reliable.