Release power to stepper motor
From the documentation:


  TERMINAL 7 Disable
  This terminal will force the winding currents to zero when tied to the step and direction controller +5V.
  
  The DISABLE input on the G213V is optically isolated and requires logic “1” to DISABLE and logic “0” to ENABLE the drive. Once it is disabled, the motor windings are unergenized[sic] and the motor freewheels. (emphasis mine)


So, feed the disable input a high.