We don't have any direct flights iirc -- they're all passing through other countries. If we imposed a ban, we'd have to do so unilaterally, as these other countries haven't stopped flights. If there were an international agreement, it might be possible.
Unilaterally, it might stop some people -- it might indeed! But other people who have passports from elsewhere but have been in infected countries may very well conceal their status and slip in anyway. In that case, they'd be very reluctant to go to the hospital and more likely to stay home until deathly ill, infecting their family and neighbors.
It may very well cause much MORE Ebola in the US to stop 9/10 potentially infected people but drive the 10th into hiding -- especially since frequently immigrants live in crowded apartments with multiple family members and many crowded apartments nearby.