unwanted posting

Hi,

I have a Joomla 1.5 site with Ninjaboard and have noticed spurious unwanted postings from Poland etc when I create a test post you see postings appear at the bottom of the page in foreign language. I get random users creating accounts and I think they are adding "invisible" posts to my website probably as some sort of under hand SEO activity. Where does Ninjaboard store it's posts so I can go have a look and clean it up. Will installing spam fighter help against the types of postings.
Trouble is you can't see the postings when you access the forum through the normal methods, but you know these people are getting in an adding content somehow.

TIA for any help.
Cheers, Del-Boy

Update:
I have found that the Ninjaboard uses one of the existing mysql databases on the backend of my site and I used an admin tool to clear out all the "hidden" posts. I have exported some of the entries if anyone wants to see them.
So, now the db is clean, I'm interested how they can add these posts without them showing in Ninjaboard? There was around 50 posts and yes they were black hat SEO posts.
You can only see them in mysql. I'm trying now to work out how to close the back door. I have seen various users created in the user manager, but sometimes they are not even activated. There is no delegated permissions or remote access allowed onto the mysql database, so they must be coming in via the front end of the site somehow.
Would be interested to hear if other people have had this issue at all and what they have done to combat it.
I wonder how prolific the issue is??
I'm new to web admin but am picking it up and learning!!

Del-Boy