I may be misunderstanding, but if "keep uploading the same document" is an issue, retain hashes of recently-uploaded documents and refuse to handle anything seen too recently too many times, unless being paid.
Sorry, to be more precise, "same document" could mean the same content but in different formats. OR just slightly tweaked to result in a different hash each time. Harder to abuse for sure, but still has holes.
Any sufficiently-determined (and smart enough) user will find a way around any such check; this is just meant to make it a bit harder to do the wrong thing so that some people will stop trying...