Regulators had been investigating allegations by AP (APET.LG) that Nova Finance and Securities Ltd, acting on Dangote's instructions, had forced its share price down more than 80 percent between Feb. 11 and March 24.
AP placed a two-page statement in a Nigerian newspaper last month saying the alleged manipulation was linked to a dispute between Dangote, head of one of Nigeria's biggest conglomerates which bears his name, and its own chairman Femi Otedola.
Dangote and Otedola are two of the country's best known tycoons and the only Nigerians on the latest Forbes billionaires list, worth $2.5 billion and $1.2 billion respectively.
The Securities and Exchange Commission (SEC) said its investigations showed that Nova had "employed manipulative and deceptive devices" and that cross-trading in AP stock had indeed lowered the firm's share price.
It imposed a one-year ban on Nova and disqualified its managing director Eugene Anenih from working in the securities industry for five years. But it cleared Dangote after Anenih said he had not received instructions from the billionaire.
"The commission did not find any evidence to show that Alhaji Aliko Dangote instructed Nova Finance and Securities Ltd and Mr Eugene Anenih to carry out any of the transactions in AP Plc purportedly done on his behalf," it said.
It also said it had referred Anenih to the Economic and Financial Crimes Commission (EFCC), Nigeria's anti-corruption police, for possible prosecution and called on the stock exchange to upgrade its surveillance of unusual transactions.
The stock exchange suspended the brokerage firm last month pending further investigation and fined AP 300,000 naira ($2,056) for publishing its allegation in the media instead of first reporting the matter to regulators.
