But free market competition isn't a negative externality, so I didn't notice that it was another question
If you want, you can say that driving others out of business is, but then expanding your own production more efficency would have to be a positive externality.
So no government intervention needs to be done in that scenario to reach an efficency market equalibrium.
But when you look at things like pollution, there is actual ways to guess what level of taxation is nescesary to reach a more efficent situation. If anything, an estimated taxation or regulation against things that have negative externalities is better then doing nothing.
Because then we would still have lead paint, poissonous pesticides ect. Those are all examples of externalities, and I think we can all agree that our economy is more efficenct with regulations against them.