Relay control of AC motors, handling turn-off kickback
My guess is that all the 10nF or 100nF recommendations are for zero-crossing TRIAC applications, and that one has to be really careful to not apply that recommendation blindly to relay-based solutions.

So, I propose the following possible solutions:

1: RC snubber designed according to the calculations in How to calculate resistor and capacitor size for snubber circuits . This protects the relay from arcing and reduces EMI problems.

2: 100nF and 100 ohm RC-snubber in parallel with a MOV. The snubber doesn't protect the relay, but may reduce EMI-problems since such a snubber forms an RC low-pass filter with a 100kHz cutoff. 

3: Just a MOV. May cause EMI-problems?

MOVs have a limited lifetime and must be sized appropriately to have enough longevity for the application. They may not be a suitable solution if the motor is to be turned off frequently. Placing the MOV across the relay protects the relay better, but means the MOV is always energized even when the motor is off. A short-circuit failure of the MOV will then start the motor.

The MOV may be changed to a bidirectional TVS diode. IEC 60950-1 (if it is applicable to the application) explicitly forbids TVS diodes for surge suppression. If the TVS is connected across the relay, it is basically connected the same way as a surge suppressor, and may thus not be allowed(?). IEC 60950-1 also states that MOV:s, if used, must be used together with "an interrupting means having an adequate breaking capacity", presumably a fuse.