In Phoca Forum, I am using phpBB project, I get spammed every hour by a lot of spams (mostly human spam). You can find a lot of information about "spam and phpBB". Does it mean, the phpBB is not OK? No, the phpBB is one of the best forum project (similar to Kunena, etc.) but because it is famous, the information about spam is well known. But I will newer rate phpBB with information, it has no sufficent spam options. :-(
Try to read the article listed above and similar article to know the situation about spam problems. Phoca Guestbook uses many spam protections and can be extended by many other anti spam plugins made for Joomla! But mostly this depends on administrator of the site which methods to protect site inputs he/she will implement. As described in the article, try to set that only registered users can leave a message, because this is only one method which can prevent from human spam (and of course this does not work 100%). The site which get input from world must be daily checked, because the 100% protection against spam does not exist.