I used to use this method, but I found it to be unreliable. Sometimes it stops working when multiple units enter the kill zone, and a unit with a negative rating (anyone who kills enough friendly units) won't be detected at all. So I made this exploit-proof method.
Trigger activated by Anybody present, repeatedly. In the condition below, replace SIDE with the side that's supposed to be protected by the kill zone, not the side it's supposed to kill. This will even kill negative rating units, and won't be broken by multiple intruders.
Condition:
(vehicle player) in thisList AND side player != SIDE
Activation:
(vehicle player) setDammage 1; player setDammage 1; hint "You violated enemy territory!"
The one thing this version of the killzone won't do (without a little more work) is broadcast a message to everyone that x player violated the kill zone. If that's really important to you, it can still be done with some publicVariable work.