As incongruous as it seems in this otherwise cleanliness-obsessed country - graffitti seems to be tolerated.
Do NOT paint the walls yourself; if you do, you could well find yourself the object of criticism and complaints. Someone won't like the way you've painted it - after all, you don't have a diploma in house painting

- and you just might find that you get charged the cost of a professional painter to rectify your 'mistake'. No good deed goes unpunished.
I hate hate hate the graffitti - but it doesn't seem to bother most people here. All you can do is to keep pestering the agent, or to try to convince the other neighbors to join with you in making a request for repainting.
Good luck - but don't hold your breath.