It does not sound like this is a new ordinance, or even an unusual one. What may be new is police attention to it. While I wouldn’t advocate an exactly Singaporean approach to the issue, I’d say anyone caught tagging must be required to clean up ten or twenty times as much including the original tagging. If they run out of buildings to clean, then start cleaning train cars.
STOP....do not make them clean train cars....I have seen many, many cars and they are actually works of art.....yes they are painted on someone elses property, but they are amazing and hurt nothing. If they're on your building....O.K. they may be damaging, but on a train car which is a blank space and shown in numerous locations, they are harmless. If I were in charge, I would put my logo in a small area at each end of the car and leave a prepared space on about 3/4 of the area for "artists"....Make the space easily eraseable, but let them have at it...Their art is amazing.