Forward Alexis Sanchez continued his revival as he set up the first goal for Antonio Candreva in the first half and added the third himself on the hour. Cristiano Biraghi and Roberto Gagliardini scored the other goals.
Antonio Conte's Inter have 71 points from 33 games while Juventus, chasing a ninth successive title, have 77.
Inter have slashed five points off the difference between themselves and the Turin side in the last three games. SPAL have 19 points, 11 adrift of the lowest team outside the drop zone, and look doomed to Serie B next season.