If you think you're getting spammed because of a weak software, you probably still have not paid attention to our Captcha add-on
which increases the automated spam from 100% to 0%. So a beer investment will save your forums purity.
I suppose spammers just know your software is not protected that's why they are bombing it. As you see on our forums which are bombed each day, the spam doesn't seep in.
Anyway if the spammers begin to attack your forums, it's only the good sign. Spammers will not post on the site which is hard to find. That only means your site or forums have a targeting audience.
It also means the forums could be found easily not only by spammers, but also by other visitors - most possibly in Google. That proves miniBB is the software which gets you visitors faster than anything else ;-)