This Event is Cancelled
londonist.com
londonist.com
However, it doesn't discourage future posters much (they'd have to know the 'cancelled' sign was fake, and if everyone knows that, they 'cancelled' sign doesn't work any more) and it doesn't do anything to clean the poster off.
Since the intent is to keep the place clean and beautiful, leaving the poster violates that. It still has to be removed, and it's still just as hard to remove it.
The club promoters are well aware of what's going on and it's a significant disincentive for them to pay for flyposting if there's a chance that it will do their event more harm than good. The council aren't trying to completely stamp out flyposting, they're just trying to make it less attractive than flyering or advertising on pirate radio. It's a rather clever economic nudge, designed simply to increase the CPA of flyposting.
Seriously, though, it's clever even if it isn't sustainable. They'll just have to remain clever and adapt. "Darwin, shit happens, I Ching, whatever man, we gotta roll with it".
I generally like to admit that I flagged the post for discretionary purposes during a conversation in the comments, but since you're being pedantic and four other users agree, it doesn't do me any harm to leave that bit out when I ask why an article is being posted on HN.
Please enlighten me?