It's practically impossible to stop people coming into a country from the infected ones, at least without massively affecting the economy and potentially shutting off essential supplies*.
People can travel (quite easily) via indirect routes, or by crossing borders illegally, so unless you can supply enough troops (in full protective gear, and isolate them) to put one every couple of meters along the borders of the countries with cases you're pretty much stumped.
Instead you try and locate people as they come into the country, question them, if needed screen them and then try and keep track of them in the country.
*If illegal immigrants are willing to hide under trucks despite all the measures we take, how motivated do you suppose someone who thinks they may have ebola and wants good healthcare to be? (the only way to completely stop people sneaking in is to cut off pretty much all traffic and travel, which will badly affect our food supplies let alone anything else).
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