I don't have a problem with cornflour on Stonehenge if it benefits a good cause. This is a good cause. I do have a problem understanding how this sort of action helps what their cause.
In terms of simple cause and effect, I can't see it achieving much beyond varying degrees of annoyance among the wider public, many who might also support their aims (that and hefty prison sentences).
It's all a bit Basil Fawlty - your car won't start so give it a jolly good thrashing with a twig. I see that JSO are aiming for outrage, but I don't see this sort of outrage achieving much. Surely spraying the homes and cars of prominent polluters would be better, only with water based gloss paint. That would be direct action. Attacking Stonehenge is indirect action, every bit as indirect as voting for the Green Party.
Stunt politics is rubbish.