You can disable sharing for an entire class, but not for individual students yet (details here). Disabling for individual students is something that we're pursuing, but it's unlikely to come before the new year. I do have workaround that you might try, but it's a little gnarly:
1) Create a new section for students you want to disable
2) Add the students who haven't turned in their permission slip to the new section, but DON'T remove them from your current section.
3) Follow the instructions I posted above to disable publishing for all students in the new section you created. This should remove their ability to publish across the board, but they will still show up in your original section.