That exact thing is the Achilleus heel of EB, just wait till the AS or Ptolies (whoever wins that fight) starts sending 3-5 fullstacks your way every turn.
The omly way to avoid it is to play on M/M (but to some extent it is still there) or keep buffers of Eleutheroi between you and the other factions. On VH/M land border means relentless war that will only end with enemy annihilation. There are many a thread with complaints on this and a few tricks to alleviate the problem.
1) Get peace, and preferably alliance, and give them 200 or so Mnai a turn (according to Ludens, I never tried it- i do not bribe my enemies to leave me alone).
2) Attack and sack border and recruitment towns, execute every inhabitant and burn all buildings, then leave it. But that leaves them so weak eventually that you might as well Blitz.
3) Keep strong garrisions at all entrances along your border. If long stretches of passable land build a Limes of forts with a missile and infantry unit in each as minimum and a full or allmost fullstack within reach within three turns of all of them, as well as at all riverpassings and passes.
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