Probably a fake, haven't been able to find another source for it. Much milder posters (like "don't vote against black") have already caused an outrage there, so something of this kind?
Fake or not, it is gaining traction and I've seen over a thousand comments agreeing with it, all from the lower class uneducated ethnic population. The ANC is currently under threat because the DA has taken the whole Western Cape and are very close to taking the Gauteng province as well. If this happens, the ANC will have lost it's two biggest economic powerhouses - so convenient that this pops up right before voting time.