So, I just build a precinct on every planet if I know a criminal syndicate is around, and that always does the job eventually.
I build a precinct once they have a branch office on a planet. It's pretty easy for the player to counter with just one building slot. The anti-crime campaign planet decision also helps a great deal. The branch offices close after a while if there is no crime. I haven't played a megacorp yet so I don't know if the AI isn't using it well or if it's just easy for an observant player to counter by design.
In my current save I still have some unfinished business after the Great Klaggian Deception...However I am reaching mid to late game in my save; and there was a bug that just absolutely bloated my species list to near unusability so I'm starting to feel like starting a new one even though this one has been pretty great.
How do necroids work? Are they omnicidal like terminator machines or can they make feds and alliances and other agreements? I'd like to get it before I continue or start a new game but I'll probably wait for a good sale.
EDIT: I want to say the species bug where the AI went insane for a short period between patches with it's gene modding new species is actually pretty awesome except for the UI not being built for that many. It's not alphabetically listed for example and without a search or narrow function it makes finding a specific species take so long as to be functionally impossible. When I settle a new station or something that requires selecting a certain species I just skim through and look for a species with a traits display that shows a huge list of positive traits; I don't bother discerning past that as there are so many.
For example the Klaggians triggered the Great Khan to attack me, and I accidently sort of wiped out that whole species as they lived solely on hostile space stations... After that fiasco was over I would like to search my galactic species list to see if there are any survivors of the raider species or more likely in my save some sort of genetic superbeing the AI created due to the species bug that is some fraction a descendant of the raider species. There only survivors I am aware of are a fleet that was out raiding during the Great Khan's short reign. They returned and now sit and do nothing next to one of the empty stations. It would be more satisfying to be able to give them their stations back but perhaps integrated or as vassals.
That way I can repopulate the raider's empty stations. Right now they are a sort of a monument to the Great Klaggian Deception, a terror attack through a wormhole from one side of the galaxy to the other on a raider's station that triggered the Great Khan. If I wasn't playing benevolent dwarves I'd fill the stations up with Klaggians instead...
I also have been using the Steam beta branch for Stellaris because it had a fix I wanted a few patches ago and I forgot to change it back to stable; so that could be why I had the species bug.