The suspension, Steffensen's second in two years, will run until August 1 meaning the 30-year-old will miss the chance to qualify for the August 10-18 World Championships in Moscow.
The 2006 Commonwealth Games gold medallist, born in Perth to South African parents, made the accusation prior to the London 2012 Olympics when Steve Solomon earned selection ahead of him despite Steffensen beating his teenage opponent in the trials.
Steffensen, who competed along with Solomon in the 4x400 metre relay in London, had also complained that AA chief executive Dallas O'Brien had lied to him about the reasons for selecting the younger runner.
AA said that an 'exhaustive independent investigation' had found no evidence of Steffensen's claims and opted for the punishment after an hour-long video link discussion with the athlete on Wednesday.
"The board took into account Steffensen's statements and also his previous three-month suspension in 2010 before unanimously determining that a six-month suspension was appropriate, considering the seriousness of the breaches," AA said.
Steffensen was banned in 2010 for abusing AA officials at a national championships and for comments he made when announcing he would not defend his Commonwealth Games title in New Delhi later that year.