There are a couple options for deleting or hiding videos from showing up.
First, if a student includes their email when submitting, they will receive a confirmation email. Part of that email includes a link to deactivate the video and hide it from showing up (the video will still be viewable by the grid owner).
The other options are only available to the grid owner. They can use their Flipgrid Admin (admin.flipgrid.com) to select a grid, select a topic, and then they should see all responses. There they can use the Actions button to delete or hide the video.