Is there a way to filter certain strings of information from being posted?

When you leave a bbcode tag open, you get a warning and it prevents you from posting until you close the tag.

How hard would it be for me to alter the language of that code to include certain other prohibited strings?

I'm a bit of a PHP novice, but I learn things really fast. The thing is, with prior projects that I took on, there have been people before me that had written guides and tutorials, but I can't find anything like that for PunBB.

So, would I be able to add some code to the existing PunBB BBCode Mod, that would prevent certain strings of text from being posted?

Thanks for any advice or insight.