You can't stop people if they know what they're doing. That's why every arrest you hear coming from "the darkweb" is due to a glitch, bad coding, or more typically, stupid users.
There is no way to catch the literally millions of people who go on sites every single week and download free child porn, nor is there any way to stop the thousands who buy drugs every week, at least without going full-fascist and shutting everything down and mandating government monitoring chips in all computers and specific internet identification to those chips.
Communities of criminals online that break the law and abuse others, like pedophile rings, have one weakness, letting someone who isn’t properly vetted into their group.
Pedophile rings
hahahahahaha those don't exist silly, those are myth.
Because often the greed for a larger community and the belief of invulnerability of services like tor, criminals will let people into their group that are there for one purpose, to catch them.
Uhhhh maybe you haven't heard, but they've fixed that issue in general.
To become a moderator or in the administration of any child porn website you now have to post a picture with a timestamp of you producing your own child porn, with your user name on it too.
Drugs, well, they haven't caught up to the times. Pedophiles are smarter than druggies.
What does that mean?
FBI infiltration is legally impossible as they cannot provide proof of "non-cop" and thus they will not be approved.