A transport scrambler (or pattern scrambler) was a device that would disrupt a transporter beam. This could prevent someone from leaving or entering an area covered by the scrambler.
In the year 2268 the Gorn shipyards on Seudath had transporter scramblers as part of the defense system. A scrambler was also activated in the Klingon Imperial Intelligence safe house in Tzoryp when the intruder alert alarm was activated. (VAN - Declassified novella: The Stars Look Down)