Two mindless examples in WKLB..
http://www.themodernantiquarian.com/post/51587
http://www.themodernantiquarian.com/post/51588
The trouble is that they are scratched not chalked. Experimented with a bit of flint and one of the (modern) marker stones outside and got the same result.
A bit of spit on a hankie had no effect.
Is it still worthwhile posting examples like this? IMHO it needs to be done if only to show that if others take the same pics in years to come and the graffiti is still there, how long this sort of defacement lasts.
Jim.