The incident occurred in the first half when he picked up a yellow card, Nagusa told French rugby weekly Midi Olympique on Monday.
"I saw this man in the crowd. He was doing terrible gestures... like a monkey... at the beginning, I did not pay attention but he continued," Nagusa said.
"I was about 10 metres from him and when I went towards him, I was wondering if I was going to jump on him and punch him but I changed my mind. I'm better than that."
Nagusa said he would not file charges, explaining that he did not want to "do anything that could ruin this man's life".
In a joint statement, Montpellier and Pau said they would do "everything they can to sanction this act that does not reflect the spirit and values of the two clubs or rugby".
(Reporting by Julien Pretot; Editing by John O'Brien)
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