This is what we have been trying to figure out. 😒 At least in part.
Here's a longer explanation to help you towards writing it. Maybe you'll be the first one to write such an anticheat.
The host creates a lobby. The clients join the host's lobby. The host checks the clients' Steam account and the clients checks the host's Steam account. This means 3 things.
1. The host can see which client cheats.
2. The client can see if the host cheats.
3. The clients cannot see if the other clients cheats.
This also means that the checks are all performed client-sided. Changing the code on your side only affects what you detect, not them. Thus, you cannot stop them from checking you unless you hack into Steam servers to validate your account information, or you trespass into your clients' computers to modify their program. Hence the anticheat is there to stay unless you figure out a new way.
(And it also means you won't be marked cheater if you play solo.)