Russian track cyclist Victoria Baranova has been sent home from the Games after testing positive for testosterone.
Cycling's world governing body, the UCI, confirmed the 22-year-old, a European individual sprint bronze medallist, would take no part in London 2012.
Baranova was one of three Russian women selected to compete in cycling at the Games, and had been due to race in the keirin.
Meanwhile, Belarus hammer thrower Ivan Tsikhan, who won the silver medal in Athens, has been sent home after he tested positive on a retest of a 2004 sample.
Belarus team spokesman Oleg Grinko said: "We have received a letter from the International Athletics Federation that we should take out Ivan Tsikhan for doping from 2004 in Athens."