For ri.ms and tinyarro.ws, we've actually seen significant reduction in spam just by banning abusive IP addresses. So far, that has had the biggest bang for the buck in terms of dropping spam considerably.
In other words, we periodically do human review and obvious spammers are IP banned. It's a neverending problem, but they don't use so many IPs that things are out of control.
If you do come up with a better approach that doesn't ruin usability, I'd love to hear it, too!
Thanks for the comment. Unfortunately what I'm seeing is that each time they submit it's with a (somewhat) different IP address. For a given link, they might be all within 173.x.y.z, but if I ban 173.*, won't there be legitimate IP addresses in that range?
Yeah-- we do specific IP addresses and not ranges. We thought at first they were all different, but they weren't. There were hundreds of different, but lots with of multiple spam posts, some every day. Group them by IP address and you should see some that are bugging you all the time.
Hard to say though-- could be a different "shape" to your spam than what we get.
In other words, we periodically do human review and obvious spammers are IP banned. It's a neverending problem, but they don't use so many IPs that things are out of control.
If you do come up with a better approach that doesn't ruin usability, I'd love to hear it, too!