I can name a few. For example Neukölln in Berlin, or some areas in Köln (like Kalk, Ossendorf, Chorweiler, Porz), or the north of Duisburg. You can find lots of references online - search for "no-go zone germany", or simply befriend a German policeman.
All these are areas where policemen are afraid to work because they may get suddenly surrounded by 40-50 people. It's true, you can still access them, but then it doesn't mean you get out easily, hence they are not accessible.
Also, I live near one of the most damaged suburb in France, one that would earn a "no police zone" easily 10 years ago; it's now liveable, so much that I look at the sky when I drive in it now.
People react in fear and absolutes, but things are fixable.
