I know these people are infuriating but it really is best just to ignore them (at least whilst they remain non-violent). The rational response was demonstrated by the servicemen seen walking past them, en-route to a colleague's funeral, who just smiled and carried on as if they weren't there. That was true dignity.
You will never persuade these people to alter there opinions through reasoning with them as normal society's values mean nothing to them. As stated previously, it is the children that deserve our pity. With the exception of the half-wit who joined as an adult, it is through brainwashing children (and enlisting the feeble minded) that they perpetuate themselves.
I hate to say this but, in a democracy, even these idiots are entitled to their opinions.