Do not smear magnesium on the boulders in Fontainebleau, but use pof if absolutely necassary.
If you aren't capable of leaving magnesium at home, dry your hands with it, but make sure no magnesium traces are left behind on the boulders.
Always clean the boulders after climbing with a non-metallic brush.
Be a nice law abiding citizen, and take your own garbage and the garbage of others with you from the forest.
Point out to others in a friendly way if they are misusing magnesium and explain them what they are doing wrong.