I think, with a concerted effort, we might be able to discourage them from repetitions. They've already half admitted it may have been a mistake.
I think their weak point is the ecological effects. I know for certain, from my anoracky entomological experience that ANY pollutant, however diluted, kills my catterpillars. Every site they consider painting has a chance of containing rare or protected flora and fauna and it's just not possible that they've tested for every possible bad effect. They haven't mentioned the ecology at all so may have not researched it (you'd need to do it over a year to be sure). So that's a worrying unknown for them, that I'll work on with a series of questions like "have you checked if it affects A,B, C, etc ad nauseam.
Also, although we'll probably never hear, English Nature and English Heritage and The Friends of the Ridgway and the Dept for the Environment and lots of others are probably ripping into them behind the scenes. How could EH disapprove of the Countryside Alliance stunt and approve of this? One of the main complaints is likely to be "why didn't you discuss it with us first?". EH have recently been opposing the construction of a new Millenium White Horse at Folkestone, on the grounds of loss of habitat.
My bet is they'll never do this again, though they'll never admit they've made such a decision.