Make Robinia die
These Robinia pseudoacacia trees are located in italy, I don't know if all of them are joined by a single root system or if there are some different plants
Robinia pseudoacacia is considered an invasive plant in many regions. Our advice is restricted because we do not know your local legal restrictions on pesticides, etc.
These plants do tends to be clonal, so the many seedlings may very well be connected. This, along with a strong tendency to re-seed, makes control difficult.
Here is a fact sheet from the Michigan Department of Natural Resources. It may help you understand and deal with this plant: