Ask HN: Favorite methods to block spam in your apps?

Lately, http://www.theprobono.org/ has been absolutely slammed with spam project postings. They tend to consist of random characters for the username, random real-world locations, and typical spammy descriptions.

I just pushed a sanity check for the time between when the form is displayed and submitted, ensuring a reasonable amount for a human to fill it out. Beyond CAPTCHA and honeypots, any other ideas?

I've also been looking for a possible 3rd party service that could look at the project description and give a "likelihood of spam". Any thoughts?

Thanks!