The incident occurred during the Champions Cup encounter between Ulster and Racing 92 at the Kingspan Stadium earlier this month.
Speaking after the match, Zebo tweeted: “I hope my ears deceived me with some comments directed my way from the crowd.”
Racing 92 said Zebo, who has represented Ireland in 35 Tests, had been targeted by racist insults during the game.
Ulster issued a statement on Tuesday which said: “Following a robust investigation into allegations of abuse directed at Simon Zebo during the fixture versus Racing 92 on Saturday 12th January, Ulster Rugby has sanctioned a life-time ban on a spectator, for breaching our stadium regulations.
“We would like to thank the many supporters who provided information to assist our investigation.
“We are all very proud of our reputation as an inclusive club and the unacceptable behaviour of this one individual is not reflective of our supporters, or the values of our club.
“We have issued a written apology to Simon Zebo and Racing 92.”
Zebo thanked Ulster via Twitter for their swift action regarding the incident.
— Simon Zebo (@SimonZebo) January 22, 2019