MOSFET Drain - Source overvoltage protection
The following circuit should work at interrupting the BLDC load and mitigating the overvoltage spike. When interrupting the load a voltage spike will typically occur due to the stray inductance between the supply and load, e.g. in the wiring. (Inductances do not like abrupt changes in current). In the circuit below, a voltage spike higher than 63V + Vgs_th (the threshold voltage of the MOSFET) will turn on the MOSFET, conducting the energy in the spike to the negative terminal and thus eliminating the overvoltage spike. You can use a standard zener diode since the MOSFET absorbs virtually all the energy.
A transient voltage suppression (TVS) diode connected directly across the BLDC terminals should also work at suppressing the overvoltage. These diodes are designed to absorb an enormous pulse of energy in an overvoltage pulse for a brief period of time, i.e. the switch off instant. You can read more about TVS diodes here.