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. (TV: Rosa) However it seemed to prevent him from displacing Rosa Parks in time, so instead he focused on changing events.