This plugin adds a security question to the Buddypress registration page.
This plugin adds a customizable security question to the sign up form. You can choose your own question and answers. Multiple answers are possible, you can have a question like: Do you like cats or dogs and have both as acceptable answers. The goal is to fool the spam bots. Change questions as often as you need, it's easy. The plugin can't stop spam altogether, but every little bit helps.