Make a trigger of whatever radius you want to protect triggered by Anybody repeatedly.
Condition:
(vehicle player) in thisList AND side player != SIDE
Activation:
(vehicle player) setDammage 1; player setDammage 1; hint "You violated the border!"
"SIDE" in the condition is whatever side DOESN'T die in the zone. If the zone is supposed to keep out everyone, change the condition to just (vehicle player) in thisList.
I've answered this question about three times, I think that means it's time to write a tutorial