If you are going to prevent people from being able to post a message because that message may or may not contain "offensive" language, then you MUST tell people what the offensive language is in order to give them the ability to change the language. If you do not tell people what the offensive language is and simply have the page return "Sorry, the content you've filled in contains information that can't be submitted" then the poster has no means of fixing their post because they have method of knowing what was causing the problem.

This is message board design 101. Hell, this is necessary even if you are designing national legislation. A person cannot fix a problem without being told specifically what the problem was.