Hello Paul,
You can select guiding star but not the star for the calibration. Calibration star CCDSoft picks automatically and it is the brightest star in the guider FOV. Of course, you can click on the star, but for calibration purposes CCDSoft just ignores your selection. Normally you get "Invalid motion" messages only during the calibration process and that's where CCDSoft gets confused. Once you've done the calibration for the night you can select your own guiding star (which will generally not be the one CCDSoft used for the calibration) and no confusion is possible during the guiding.
Regards,
Zoran