The striker's goal arrived in the 30th minute. Marco Di Vaio picked Diamanti out inside the area and after his initial shot was blocked, the centre-forward pounced on the rebound to score from close range.
Di Vaio was the creator again for Bologna's second of the evening just before the break. His cross was met by Gaston Ramirez, who back-heeled into the net from six yards out.
Fiorentina's hopes of getting back into the match were dented four minutes after the restart when Ruben Olivera was handed his marching orders for striking Diamanti around the face.
Despite not adding to their total, Bologna made the most of their numerical advantage to wrap up the victory.