The official “reCAPTCHA plugin” provides CAPTCHA support for MyProBB.

However, MyProBB has plenty of other anti-spammer features available built into the default forum:

• Trivia questions.
• Essay question: Why do you want to join this forum? Moderator approves the answer.
• Profiles are private unless made public. All profile changes are moderator approved.
• E-mail address domain blacklists.
• Number of accounts that can be created in a day are limited.

And much more.