Anubis is the only tool that claims to have heuristics to identify a bot, but my understanding is that it does this by presenting obnoxious challenges to all users. Not really feasible. Old school approaches like ip blocking or even ASN blocking are obsolete - these crawlers purposely spam from thousands of IPs, and if you block them on a common ASN, they come back a few days later from thousands of unique ASNs. So this is not really a "roll your own" situation, especially if you are running off the shelf software that doesn't have some straightforward means of building in these various approaches of endless page mazes (which I would still have to serve anyway).