One thing that might help is requiring new users to complete a small 5-minute tutorial on how the site works before they can upload anything. And/or if a certain percentage of their observations are flagged as low quality, they have to (re)complete the tutorial before they can upload any more.
I think this would be just irritating enough to discourage the massive spam dump users, and not a big enough barrier to drive away the users who might just be new and clueless.