If you're worried about USB or any other device access you've already lost. Anyone who can SEE the screen can snap a pic of the screen. Or a few hundred screen pics. And even if you strip everyone naked as they enter the building, and you scan them for hidden devices hidden inside body orifices, the fundamental issue is that information can be carried out in someone's memory, and that person is capable of talking.
Compartmentalizing who can access what may limit the range of what any particular insider can release, and reduce the number of insiders able to release any particular thing, but fundamentally people need to see the information to do their job.
Threat of prosecution can keep people's moths shut to some extent, but if you're engaging in illegal or immoral activity then sooner or later some insider is likely to decide to "do the right thing" even if it means huge self sacrifice.
As others have indicated, maintain goodwill and loyalty. At a minimum maintain some level of respectability for organization, and some level of respect for your employees. That is the *only* thing that can protect you against the threat of a self-sacrificing insider trying to "do the right thing".