Log in as an administrator in your tournament and be sure that the 'Use Separation Criteria' flag in SETTINGS is set to 'yes'.  Go to SEPARATION CRITERIA in your left menu and change the drop-down menu to 'Exclude Field From PreReg'.  You can then go back to SETTINGS and set 'Use Separation Criteria' back to 'no'.

Click here for a video that may be helpful.

Thanks