Jammer support would not help UAVs very much, as to prevent the enemy hijacking their UAVs they would need to jam the control channels for the UAV.
For most of its flight, except perhaps takeoff and landing, most UAVs fly on what equates basically to an autopilot, with automatic control. You don't fly the UAV manually... most of the time you just shift waypoints or command it to fly a certain pattern of flight. If something on the ground is detected that needs to be tracked then you would put the UAV in orbit mode where it would fly a fairly wide circle so as to remain in the area of the target... the UAV operators don't fly the UAV very much except sometimes for takeoffs and landings, and they might command the UAV to fly orbits around an area, or they might order it to fly in a circuit or particular search pattern for certain mission, but the UAV operator is normally operating the cameras and sensors rather than flying the aircraft.
That is why they are so vulnerable... when you are looking at targets 5,000m below you with a powerful telescope you might not notice a helicopter flying along side or the tracer rounds zipping past you... the first thing you will likely notice is things start failing as they are hit... the main problem with UAVs is awareness of what is going on around the aircraft... including incoming missiles and other threats.
The famous footage of the Mig-29 shooting down the Georgian UAV was largely because the Mig-29 came in and circled to identify the target before falling back and firing.
Understandably enough if the target is unmanned then ordering it to land is pretty pointless, so shooting down is you only remaining option.
Even small stealthy UAVs are easy to detect because they transmit live video feeds as part of their role. They probably use burst transmissions to reduce the chance of detection, but clearly the Iranians managed it, so we can assume that with the right equipment the Serbs could and the North Koreans could too. (As well as China and Russia).