I always joke with my users each time their machines face problem that they should stop surfing porn.
Recently I attended to a machine that is down with virus. I said the same thing to the user but this time it's for real because there's porn everywhere up to the point I think he's needs some help.
Whatever it is, to me it's simple. What you do on your own time is your own business but keep it personal. In other words, never use company's resources and properties for stuff like this. Not only it is not professional, it violates policies even in the most liberal corporate organisation unless the business is porn.
He's given a warning and asked to report back with the violations rectified. I rather settle it at this level first but he's already in my black book.