I don't particularly like the idea of harassing possibly innocent students. Innocent students might be under pressure with no proof of innocence, since they assumed to be guilty. They might do a cost/benefit analysis and admit guilt. Also if cheating is really such a concern, why not just handle it in a similar way to the american GRE's or SAT's ?
The only foolproof way to "convict" someone of cheating is to have (several) people witness/record the incident. If a student accused of cheating categorically denies everything, they're usually in the clear (although faculty will say nasty things in private).