Mosquito repellents have some effect against black flies, but the effect will rarely last for more than a few hours.
In the countries where black flies transmit serious diseases, the streams in which the larvae hatch are treated with insecticide. This, however, is not an approach that will be used in Denmark, where the black flies are not suspected of transmitting diseases. The vast majority of larvae and pupae stick to aquatic plants.
If you cut the aquatic plants in the streams at the right time, before the black flies hatch, it can sometimes help the problem.
The streams, in which the black flies live, should not be heavily contaminated, but a certain amount of organic matter in the water is necessary.
Cleaning the water to make it poor in nutritional value for the black flies is a possibility.