According to this survey 58% of Americans think there should be a travel ban on flights from countries where the Ebola has broken out. The survey doesn't make clear what a travel ban would mean, but assuming the USA has no power to ban flights that don't start or end in the USA, how many direct flights are there to the USA from the only countries currently experiencing widespread transmission: Liberia, Sierra Leone and Guinea?
A quick search from the capitals of the three countries to NYC or MIA shows no direct flights. Almost all of them stop in Morocco or somewhere in Europe, before continuing on to the US.
However, and ignoring how effective a travel ban may or may not be on stopping Ebola, what it would consist of is written up quite well in this National Geographic article. It would prevent travellers symptomatic who have been in those countries from being allowed to travel to the United States.
Of course, symptoms take 2-21 days to appear (same article)....so again, it may not be as effective as hoped.
There are no direct flights from those 3 countries.
There are direct flights from Senegal and Nigeria, and these 2 countries have had a few cases of Ebola, but they are all of people who were in contact with people who've come from the 3 outbreak countries, the transmission is under control, and it's not spreading in the general population.
The fact is, U.S. airlines don’t fly to the countries with ongoing Ebola outbreaks. Delta used to fly to Monrovia, but stopped in August. Today, only Delta and United offer direct, nonstop service between the U.S. and West Africa, according to Airlines for America, a trade group for U.S. airlines—Delta to Lagos, Accra, and Dakar, and United to Lagos alone.