I don't.

Criminals are minority of the Nigerian population, but that minority spends an inordinate amount of their energy trolling online and looking for new venues that regular Nigerians are not likely to visit.

Therefore if you're a US based and targeted company, the majority of YOUR Nigerian traffic IS likely to be fraud. It isn't most of the fraud that you've got to deal with, but it is likely to be so ridiculously obvious and easy to deal with that you'd be a fool not to.

(Of course this is widely enough known that Nigerian scammers use proxies to hide their origin.)