Click on Stop.
A separate window is displayed asking if you want to continue calibration or exit. If something obstructed the calibrator puck from reading the circle, you must exit and then restart calibration.
Calibration does not halt until the current color has been completely measured. For example, if you click on Stop while the Red measurement indicator is at 24, calibration will continue until Calibrator Tool is finished measuring Red (when the indicator is 255).