What the other comments said you basically just guard them by patrolling and now more commonly with small portable drones as well for harder to travel areas.
Many land borders are actually not very actively guarded because climbing over mountains and ravines isn’t that easy which is why in those areas it’s just barren or wildlife only and barely anyone lives around the border. Areas you see having border guards and customs are easier areas for people to travel which is why there is more security and often fences and walls too to prevent illegal travelling.
By actively guarding the border.
Obviously that isn’t perfect, and obviously people get through. But if a country really wants to strictly guard their border, all it’s takes is paying for more border guards and better border infrastructure.
That’s the only way to stop it, to have some sort of guard/law enforcement there to stop and arrest people crossing illegally
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