Zebo, who plays for French club Racing 92, tweeted after Saturday's game: "I hope my ears deceived me with some comments directed my way from the crowd".
Zebo's reference to "Django" is a tip-of-the-hat to Quentin Tarantino's n-word riddled, double-Oscar-winning movie from 2012, depicting the struggles of an African-American slave, in pre-Civil War Texas.
"Racing 92, does not want to stigmatise the public at Ulster Rugby as a whole", said the statement.
IRISH rugby worldwide Simon Zebo has said he was on the receiving end of comments that were "not on" from the crowd at Belfast's Kingspan Stadium during a weekend match against Ulster.
"The insults suffered by Simon Zebo are the actions of only one or two individuals", their statement added.
Ulster head coach Dan McFarland has appealed for information.
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Democrats were already seeking to question Barr about the memo, which he sent, unsolicited, to the Justice Department in June. He also expressed concern about Barr's memo and predicted it would be the "first line of questioning" for many on the panel.