Creating a Blacklist for your Comment Moderation
The Photo Contest App utilizes the Facebook Comments Social Plugin to allow for commenting on individual photo submissions from the Vote Page of the contest.
Each photo uses its own unique comment box plugin, so in order to moderate the comments with something like a blacklist for certain terms, you'll need to create a custom app to use for the offer (read more about using custom apps for your offers here).
Then, you can set a blacklist for the custom app through Facebook's Comment Moderation Tool. In the Facebook Comment Moderation Tool dashboard, simply select the custom app you're using for the Photo Contest and set up your blacklist (this is all done the same way as if you were setting up a blacklist for your Business Page), and it will customize the blacklist for all of the submitted photos' comments.