They got to somehow take this round offline aswell, their is absolutely no other way to combat cheating. Expensive? Yes but then looking at last year's cheating, if they want good teams they got to spend more on testing. Any of the the online cheating prevention tactic would fail as they are failing in online OAs nowadays.

Exactly, if people can cheat with all the screen and camera proctoring in OAs, there is no way on earth they wouldn't cheat in the prelims. I hope the organizers do something about it, maybe increase the participation fees but conduct the prelims offline?

I really don't wanna pay 1100 just to lose against some cheaters :/, hopefully they do something about it.

u can try for example hosting hubs for the contestants

each contestant is assigned to the hub closest to them

and at the hubs the computers are connected by a LAN, so they don't have access to the internet

I think this is the best bet specially that u can't have all contestants in one place as there probably isn't a place they'll all fit

or u guys do the same as Egypt where every day there's a contest for a specific amount of students, also onsite

and then they select the top teams to participate in a final day

If the IICPC can achieve such remarkable success in detecting and removing cheaters, I expect ICPC to match or even exceed the quality of plagiarism checks performed by IICPC. One notable difference is that the IICPC is hosted by college students.