There's an easy fix to stop, or at least massively reduce, this sort of thing.
Just introduce a law saying that anyone buying spray paint has to be at least 21 years old.
Yes, a few kids will pinch spray paint from their parents sheds or pay irresponsible adults to buy it for them, but the availability of spray paint for the demographic that does this sort of thing (i.e. kids), will be massively reduced.
It really, REALLY isn't that simple at all. The legal age for spray paint is 18 currently.
A large number of the vandals (the "serious" ones who do the murals) are not children as you perceive them. They're between the ages of 20 and 40. To them, it's a hobby. They take delight in creating these murals in places, and they seem to place more value on some than others. I know that retired carriages up here get absolutely smashed (stuff all security, sitting ducks), because they want to get photos of their tag on a train. They don't care if it's not going anywhere, it's the "frame" they put it on. Why trains? I don't know. They usually photograph their "work" as soon as it's done, so they have a record for it, but I still object to seeing it paraded around the internet and on the railway networks. It lets other vandals see it, and gives them street cred, or whatever.
I'm a massive hypocrit though, I don't care if they want to tag the Pacific National coal hoppers in NSW because they're pretty grotty and ugly anyway, so what's a bit of colour? I object to seeing it on passenger rolling stock though, because it creates a perception of danger - trains covered in graffiti seem less safe to those that are spotless. No logic behind it (the "serious" vandals only care about their "art", it's the thugs that scratch/smash windows that are more likely to smash a face), but there it is.
I especially object to seeing it on heritage rolling stock, because they're hurting someone else's hobby, and that's pure selfishness.