Comment by piggg
Blocking on ja3/ja4 signals to folks exactly what you are up to. This is why bad actors doing ja3 randomization became a thing in the last few years and made ja3 matching useless.
Imo use ja3/ja4 as a signal and block on src IP. Don't show your cards. Ja4 extensions that use network vs http/tls latency is also pretty elite to identify folks proxying.
Some of the bad actors, and Chrome, randomize extensions, but only their order. I think it's ja3n that started to sort the extensions, before doing the hashing.
Blocking on source IP is tricky, because that frequently means blocking or rate-limiting thousands of IPs. If you're fine with just blocking entire subnets or all of AWS, I'd agree that it's probably better.
It really depends on who your audience is and who the bad actors are. For many of us the bad actors are AI companies, and they don't seem to randomize their TLS extensions. Frankly many of them aren't that clever when it comes to building scrapers, which is exactly the problem.