Well, I had an idea, not the greatest or the best, but I think it'll stop some double posting and repeats over and over again. The idea-
When uploading bigger and more complex stuff (i.e. mods and texture packs), the notice at the top saying that remixes/other stuff without permission should be about 2x bigger for maybe 30 seconds right after you click "Upload a new ______", and be the only thing there for those 30 seconds. I feel as if this won't stop the dedicated plagiarists, but it'll at least drive away the town fools. I'm seeing happy-go-lucky not-meaning-harm people uploading modpacks, DokuCraft remixes, Painterly Remixes, etc. and I feel as if this might slow them down.

Oh, and add in to the warning- "If you have ANY doubt about whether or not it's allowed, it isn't."