[brudgers]

By default, any site that allows users to share content will devolve toward an attractive nuisance [0]. Like any security issue, passive measures are a Maginot Line awaiting blitzkrieg, even all the resources of a Google or Facebook aren't enough to automate all these things...they depend on communities to report issues [e.g. webmasters for Google]. And that's the only defense in depth: community.

"Everybody who signs up" isn't a community. There has to be some higher order interest...and what you're finding is that unfortunately the higher order interest of the community for your site is child porn.

There's no fixing DNS. If child porn is not what you want, your site is broken. Shut it down. The sort of users you want don't either don't care enough to keep out the bad or are overwhelmed by it's volume just as you are. They are or will be moving on. You have my sympathies.

Yeah it sucks but you have learned some things:

  1. Community is the hard part.
  2. Technology is necessary but not sufficient.
  3. You can build something that scales to the point where
     it becomes useful to a community.

Consider this version 0.1. You've gotten feedback and that says that the product (not the code) has failed by your definition of "fail" because it has not attracted the market segment you want. You have a platform from which to relaunch.

Good luck.

[0] https://en.wikipedia.org/wiki/Attractive_nuisance_doctrine