Dealing with voltage spikes in BLDC motor drive
First ad a large capacitor or bank of more capacitors. The zener will probably burn out very quick because of large amount of kinetic energy that is converted back in electricity. Your device needs to absorb the total amount of the energy generated during quick deceleration. Usually such device exists in servo drives named as braking resistor. This resistor is switched on by help of the MOSFET when the voltage rises above a defined threshold and it is switched off when the voltage drops below the threshold with a hysteresis. 
If the capacitor bank has enough capacitance then the voltage rise will be smaller.