^^ Impossible under international law.
We can't ban flights from any country , international air transport regulations see to that. We can't refuse emergency medical treatment either, and the 'health tourists' make sure that their condition is so advanced / serious that they have to be taken direct to A&E .
As said the Nigerian government are to blame. They issue a 'fit to fly' certificate to anyone who can pay the cost of the bribe. Women are normally actually in labour by the time the Nigeria Airways 'Lagos Shuttle' lands at Heathrow. Nigeria Airways flights from Lagos routinely have ambulances waiting on the tarmac ready to transport patients direct to St Thomas or Guy's Hospital. As a medical emergency they don't even have to go through customs or immigration.