The incident took place as the AC Milan striker ran laps around the pitch at the national team's base near Florence on Wednesday.
Balotelli shrugged off the verbal abuse, but later told his teammates that it was "unbelievable", adding that such an incident could only occur in "Rome or Florence".
"It was unacceptable behaviour on the part of the individuals who, by their actions, have demonstrated a lack of class but we have to put it into context. The important thing is that there were only a few people involved," Abete told reporters.
Nardella added: "They were unacceptable insults hurled at Balotelli. It is even more serious as it came from a young fan. Florence is against racism and promotes respect, education and culture."
Italy have been placed in Group D for this summer's World Cup finals, along with England, Costa Rica and Uruguay.