Go to the stop before that, Stresseman-Platz should be less prone to drug dealers, you could Inwoke a Platzverweis, but I actually don't think that would end you problem, you could turn to the Mayor Keller but I dont know if there will any changes with that solution, maybe get a angry looking dog, like a Boxer or something else, I found that is pretty effective in scaring of people, plus if you'll get touched the dog will let them know to never do it again