Kevin's suggestion is perfect, after reading the topic he linked, select the "Q&A Captcha" from the Spambot Countermeasures in the ACP -> General tab.
How to configure Q&A
ACP> Spambot countermeasures> Installed plugins> click the dropdown box and select Q&A (it will be greyed out)
Click the configure button then click add.
Add your question and answer (you need a Q&A for each language you have installed) click submit.
Click 'back to previous page' then click 'back'
Select Q&A from the dropdown box (again)
Click submit at the bottom of the page.
Also, I would suggest disable guest posting, make passwords have mixed case, numbers, characters, etc. & have the account activation by User, then authentication by email is mandatory for any chance to post a message. There are other measures you could take as well, but this is the best option. On one of my sites, I've just gone from 800-1500 spam per day to NIL, after upgrading the phpBB version.