Kovago won silver for Hungary at the 2004 Athens Olympics and was considered a medal contender at the London Games as well.
The Court of Arbitration for Sport said Kovago "refused or failed without compelling justification" to provide a doping sample last August.
The court upheld an urgent appeal by athletics' governing body, the IAAF, after Hungarian anti-doping authorities cleared the athlete to continue competing.
The reasons behind the court panel's verdict will be published within weeks, the court said.
Kovago's ban comes shortly after another Hungarian discus thrower, Robert Fazekas, was removed from the Olympic team for doping.
Fazekas tested positive for stanozolol, the banned steroid used by sprinter Ben Johnson when he was disqualified after winning the 100 metres at the 1988 Seoul Olympics.
Fazekas has been in trouble with anti-doping authorities before. He initially won the Olympic title in Athens in 2004 but was disqualified for a doping offence. Kovago had finished third but was upgraded to the silver-medal position in the standings.
The 2004 Olympics gold went to Virgilijus Alekna of Lithuania.