You could use skript and check rhe distance.
#plugin
ashen valley
lament robin
World border, region border? or what?
If you explain what you need better, you'll be more likely to get help. I'm not really sure exactly what you're asking for
If you're just trying to stop players from entering safe-zones. I believe the plugin https://www.spigotmc.org/resources/anticombatlog.94540/ has a setting that allows you to do that. Or one of the other combat logs. I know I've used that setting before, it sends barrier blocks client side to the player so they're physically unable to go through the wall without hacking.
