Neural restrictor

A neural restrictor was a brain implant that could stop an individual from murdering or causing harm to another living thing. Prisoners released from Stormcage Containment Facility, such as Krasko, had these implanted into their brains before being released into the outside world.

Krasko found a loophole in the form of a temporal displacement weapon, which sent those it hit to a different time zone. As this did not involve physical harm, he still had this option to get people out of his way. Krasko planned on using the weapon to displace Rosa Parks to change history. Since she would be thrown back through time, Rosa would not be physically harmed, so the restrictor would not kick in.

When the Thirteenth Doctor confronted Krasko and destroyed his vortex manipulator, he tried to strangle her, but the restrictor kicked in and he was caused physical pain. He was visibly frustrated he couldn't injure the Doctor as a result. (TV: Rosa)

Stormcage didn't always do this to ex prisoners. River Song was quite able to murder on a whim. Implying it's only for mass murderers. (AUDIO: The Furies)