This is the focus of intense research into prevention of contamination and into breeding 'aflatoxin resistant' crops for example. Several studies have used harmless bacteria and even isolates of Aspergillus itself from a variety of locations, some with very good results.
New research from China uses a novel approach to the problem and has isolated a bacterium that it considers could effectively reduce the growth of Aspergillus on peanuts. The bacterium is isolated from a novel location (the sea) in the hope that the antifungal activities it may possess will be distinct from the earlier studies - they hope to find new mechanisms to defeat Aspergillus contamination and not to repeat that which had already been studied albeit in different ways.
Marine Bacillus megaterium was used to compete with Aspergillus in laboratory experiments on stored peanuts and subsequent contamination by Aspergillus monitored. Results showed that B. megaterium inhibited growth of Aspergillus quite significantly, and also that growth media in which B. megaterium had been grown but which all the bacteria had been removed was also active. This suggests that B. megaterium is secreting an antifungal substance, possibly one that is novel and not previously known.
Watch B megaterium grow here