My site must be popular, someone just put spam in the tagboard

I already removed it, but when this happens more frequent then this one time I need to do something about it. I already have been looking for a way to prevent this before. I was looking for a PHP script to create those images readable to humans, but hard for machine to prevent this kind of posts. But I haven’t found one yet. So if someone has a site where I can find one, please let me know or post a comment (while it still is possible ).