Cleaning graffiti does prevent graffiti. Since DPW has been doing a good job in cleaning the graffiti in the areas that I report, the tagging vandals are doing less graffiti on the same blocks.
Reporting works for two reasons. The graffiti is cleaned. Less new graffiti.
My sense is the second reaction, - less new graffiti, only begins after about the third cleaning something like being cleaned quickly for a month,