Chances are the officials don’t want to stem the problem. $18B is a non trivial amount of money to inject into a country like Nigeria with a GDP of $448B (2019). No sane politician is going to purposefully cut off 4% of the GDP, especially when the costs are externalized to another country.

18B? Just how many of persons doing this in Nigeria could there be? I somewhat doubt that the figure can be that high. That is millions of individual cases, which would mean that there is thousands or tens of thousand people involved and even more with processing the money on the way...

Nigeria is the largest country in Africa. The population is ~200M (~60% of the population of the US). 1M people is only 0.5% of the total population of Nigeria, so it is not a stretch to imagine that number of people engaged in cyber-crime. When you consider that Nigeria's population skews young, and the country is filled with a lot of very intelligent, well educated people, who don't have a chance of landing a six figure salary working for a FAANG company, it does not seem unreasonable to expect people to make a living doing something that is not totally ethical (even people with all the opportunities in the world go on to work as corporate consultants).

If withdrawal of aid isn't sufficient you can use sanctions against the nation in question or even against nations that opt to trade with that nation. It logically wouldn't take much to tip Nigeria into a further clusterfuck.