It seems that Nigerian officials are not fans of the movie "District 9".
While it's true that Nigerians are portrayed as crime lords in the movie, running the underground market cat-food and weapons market (not to mention prositution), I think the thing that really irked the officials is the naming of the main "baddie": Obasandjo sounds a lot like former Nigerian President Olusegun Obasanjo. And it doesn't help that he's a superstitious megalomaniac that wants to eat aliens to inherit their power and control over alien weaponry.
Sadly, this type of thing really happens in some parts of Africa, including South Africa. (It's called "muti killing"), so there is some real-world inspiration for the crim lord's behaviour. Add to that that a large amount of crime in today's South Africa is due to Nigerian gangs, and you can see that to a large extent the writer/director was just drawing on current events. You can read more about the director's reasons for making the crime lord a Nigerian and referencing muti, in the interview here.