On the signup page for the site one of the anti-spam questions I got was "who was assassinated?" with Gandhi and Martin Luther King as the two options. As far as I know both of them were shot dead, so am just a bit puzzled.

Well, I don't know if this is what the question was going for but:

Martin Luther King died of a heart attack in 1984

Martin Luther King Jr was assassinated on April 4, 1968

So if the question expected you to notice the missing Jr it's awfull tricky for a bot/not bot determination.