Ah apologies, I has that messed up in my head, I thought you just wanted them not to count towards a grade, not that you didn't want them to show up at all. That definitely does add complication to this...
I know @JeffCampbell is helping you with his approach #1, but even though it might be workable for your specific assignment setup here, you may have other teachers unhappy with students potentially continuing that kind of attaching/submitting behavior later on in their academic careers. You'll see quite a few posts here in the community from teachers frustrated that students are using that method to try to bypass regular submission deadlines, restrictions, etc...
I do have one other suggestion, which may or may not be helpful. Instead of using as assignment, you could create an ungraded discussion for this and have all the students post their PowerPoints as attachments to a reply. That could potentially even save you the work of downloading and compiling them all if the goal it to really just have the slides available to all of the students.
-Chris