Is looking through individual browser history part of your job responsibilities? I understand you are in IT, but that doesn't mean you are required to monitor everyone's internet usage does it? If it does, why not tell the MD that this guy is looking at porn (which you did) and then explain to him that this guy is an elder and you need to report it to the congregation. Not for work purposes, but for the congregation. You can do this to get cover from your employer......Then do as Georgie Girl suggests and meet with him and two other elders, and you might ask to do it at your office, and invite the Janitor to give his side, so that the elder in question can make the accusation of "It was the janitor" in front of the janitor.
It seems you have the right and responsibility to do something, but be deliberate in what you do. Protect yourself and your job first.
Just my opinion.