As hard as we try, we are never going to eradicate cockroaches, rats and mice. All we can do is work on keeping their numbers down to a level we can live with. That means taking preventative measures like laying down baits and spraying the house on a regular basis, as well as swatting any that appear from time to time. It's a constant, ongoing process, and it requires vigilance and a small degree of effort.
Vandals are the same. Sadly, they will always be with us. However, that doesn't mean that we should just live with the results of their dissolute lifestyles. We should try to reduce the number of active vandals to the lowest level possible (given the finite resources that we have), and also reduce their output as much as possible. On the flip side, we should also increase the tempo or rate at which their handiwork is removed.
If we have 50 active vandals putting up 10 tags per week each, we have 500 new tags per week in a given area. If we can reduce the number of active vandals to 25 through pro-active policing and perhaps some useful social work, then that's a good start - it will drop the weekly tagging output to 250. If we can then limit the active vandals to putting up 5 tags per week, we are down to 125 fresh tags per week. A reduction in tagging could be brought about through reducing the number of "opportunistic surfaces". Vandals attack the "low hanging fruit" in most circumstances - the soft targets. A bit of redesign may be necessary to make targets harder or riskier to hit. Those measures won't deter the most determined vandal, but they will deter the wimps.
Increasing the rate or tempo of cleaning is all about systems redesign. If you ask me, Canada Bay Council has a good system for removing graffiti. I say that purely based upon the results that it produces. Telstra has a system that sucks, as does Australia Post. Energy Australia and the RTA are getting better, but they still lag a long way behind Canada Bay. Canada Bay removes graffiti in 1-2 working days; Energy Australia and the RTA take between 2 weeks and 2 months. They have to improve their tempo by an order of magnitude. Telstra on the other hand needs to develop a completely new mindset.