Blocking Amazon, Huawei, China Telecom and few other ASNs did it for me. You should be able to cut, sort -u your log files to find the biggest offenders by user

Blocking Amazon, Huawei, China Telecom and few other ASNs did it for me. You should be able to cut, sort -u your log files to find the biggest offenders by user-agent if they truly obey robots.txt.