Recently a former member of this group (former because the other members consider his messages to be too extreme and detrimental to the goals of their cause) attempted to pay for a boycott Israel advertisement to be displayed on the side of city buses. This is a link to the advertisement:
The proposed add says: "Boycott Israel, Boycott Apartheid"
There is also a poorly drawn cartoonish image of a spider with a skull for its head, holding in its hands bones.
The ad was rejected by the government body that oversees the public buses. They justified banning the ad because they have a policy against accepting adds that "Defames or is likely to hold up to scorn or ridicule a person or group of persons."
I guess in a certain light its pretty obvious to see that this ad defames Israel, and by extension Israelis, by the very fact that it calls for a boycott against Israel. But perhaps you could go a step further and argue that the image is not dissimilar to images that were used by Nazis to drive public opinion against Jews in 1930s Germany, and thus constitutes defamation of Jews in general.
Seeing the arguments that people have been getting into over this incident, typical pro and anti Israel propaganda notwithstanding, I got to thinking about what kinds of speech are acceptable in the public sphere and in the advertising sphere as well. When should speech be curtailed? Is defamation enough of a reason to prevent someone from saying something? At what point is it OK to censor speech? Should there be different standards for printed media and spoken media? Is it OK to have special rules for advertisements that are different for speech in other media? Should government entities be given discretion to decide when a particular image or phrase amounts to defamation given that politics can heavily influence someone's opinion, and that such authority could potentially be used to stifle political speech of political opponents?