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.