The X2 motor is rated at 350W, so assuming you are 110v AC mains the motor in use will be drawing 350/110 or 3.2amps.
A 10A relay sounds ok if that is the largest you have. If the motor was at full speed and at 110v 3.2A the relay would remove the AC power and short a 33 ohm power resistor across the motor leads. There is not a huge amount of energy stored in the motor and spindle X2 spinning mass so something like a 50W resistor would be plenty.
It would take a little experimenting, and you may find you can safely use a resistor lower than 33 ohms, to give even faster braking.
I also suggest using resistors in parallel to make the 50W resistor to improve their peak current rating. If you need it there are resistor calculators online to easily calc the value of multiple resistors connected in parallel.