What is the fine-grained topic of the following text: Last year in Amsterdam, the Venserpolder metro station underwent a real attack on a subway in November last year: “A group of twenty people stopped the subway by putting bicycles between the doors and within 5 minutes those unrecognizable boys had completely rummaged through the entire subway system.” “For many years MAVRO has been the partner for the GVB and ISS Cleaning Services in the fight against graffiti, and MAVRO also supplies a wide range of other cleaning products that are used to clean the subways, trams and buses. Only return the topic.
Vandalism and graffiti cleaning in Amsterdam's public transportation system.