Perhaps you want to prevent a Competitor from being auto-assigned to her mentor, who also happens to be a Judge at this competition.
There are two ways to do this. Both must be done after the Judges and Competitors have been entered.
First Way: Modify Judge Profile
Navigate to the Judges tab. Find the Judge in the list and click on their name. In the popup, type the name of the Competitor in the box labeled "Excluded Competitors." Click Save to save your changes.
Second Way: Modify Competitor Profile
Navigate to the Competitors tab. Find the Competitor in the list and click on their name. In the popup, type the name of the Judge in the box labeled "Excluded Judges." Click Save to save your changes.
You can do this before or after RocketJudge's auto-assigment algorithm has assigned competitors to judges (more on that here).