Q.How to get rid of Brambles permanently
What is the best way of removing brambles and prevent them coming back?
If you are looking at a large area, probably you're going to want to use an herbicide. I expect a post-emergent will be necessary for the woody plants. You're also going to be looking at some continued treatment and digging for a couple of seasons. These articles have some more information: https://www.gardeningknowhow.com/plant-problems/weeds/using-herbicide-in-gardens.htm
https://www.gardeningknowhow.com/plant-problems/weeds/post-emergent-herbicides.htm
I don't know exactly what kind of home gardening help you have available in France, but if you contact the nearest botanical garden, they ought to be able to direct you to sources from whom you could learn of the products available in your area.
