While larger groups may decide to opt for third-party spam control, we were looking for ways to make Open Outreach sites better-protected. We were also sick of all the spam generated on both openoutreach.org and chocolatelilyweb.ca. On these two sites we’d tried many different options over the years but none seemed to help much and each new solution seemed to bring new issues.
Then we moved to the Riddler module and for us it has made a huge difference. Our sites are pretty low traffic, but personally not having to sort through spam comments—whether just annoying or downright objectionable—has made it a welcome addition to our repertoire.
Open Outreach ships with Riddler, so if you want to use it all you need to to is add some questions and then set it as the default. See detailed instructions on the user documentation page about Captcha.