> By that definition, not removing graffiti (aka doing nothing) is not encouragement, [...]

"Doing nothing" results in encouragement in the broken windows theory:

  Under the broken windows theory, an ordered and clean environment, one that is maintained, sends the signal that the area is monitored and that criminal behavior is not tolerated. Conversely, a disordered environment, one that is not maintained (broken windows, graffiti, excessive litter), sends the signal that the area is not monitored and that criminal behavior has little risk of detection.

In my understanding stopping encouragement is maintenance work, while starting discouragement is social work. And by doing the (simpler) maintenance work a (costlier) social work won't be necessary.