Bluff them out. Call in the suspected cheaters, sit them down, and say, 'anything you want to tell me about using chatgpt during the exam?'

Then run a kgb master class of interrogation on them from there.

if you secure a confession, you can then give them a wise lecture on how they are cheating themselves or take another approach.

Our head of year at school bluffed multiple confessions out of me and my friends; she'd never say why she had called you in, and we'd often confess to something else we had done, and she would get a double whammy confession.