Psi-screens are devices that prevent telepaths from reading the wearers' minds and also cancels out a telepath's mind-reading abilities if worn by a telepath. They are frequently used by ghosts and their instructors during training, enabling ghost trainees to concentrate (as they won't be reading minds at the time).
Psi-screens, which are so rare that it was harder to acquire than a nuke from the former Confederate Army, come in various versions, including touch-activated ones (which are lower-quality) while newer ones can be activated without touch. Some psi-screens can produce an effect which inflicts pain on telepaths; these can be used to keep telepaths in line. Higher-ranking people get to wear more powerful devices, and at least some are adjustable.
The device is harmful if worn for more than seven hours, causing memory loss, brain damage and paranoia. The device has a greater effect on the brains of psychics than on non-psychics. Some wearers undergo brain scans to check for damage. Some telepaths sleep with them on to avoid reading others' dreams as they sleep.