Tournament organisers announced a four-week ban for the New Zealand international following his appearance before independent judicial officer Jean-Noel Couraud in Dublin on Thursday.
Afoa was cited for a dangerous tackle on Munster full-back Felix Jones during the second period of last Sunday's Heineken quarter-final in Limerick.
Couraud heard evidence and submissions from Afoa, who pleaded not guilty to foul play.
The prop was accompanied at the hearing by Ulster rugby director David Humphreys, team manager David Millar and legal representative Donal Spring.
Couraud upheld the citing complaint and found that Afoa's offence warranted a red card.
Afoa, who has the right of appeal, cannot play again until May 7, nine days after Ulster tackle Edinburgh in Dublin for a place in the May 19 Heineken final against Leinster or Clermont Auvergne at Twickenham.
In a brief statement, Ulster Rugby said it was "disappointed" with the outcome of Afoa's hearing.
"John Afoa, himself, is very disappointed, particularly in view of his outstanding disciplinary record where he has never previously received a yellow or red card during his professional career," the Irish province said.
"John looks forward to returning to play after May 7."
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