Rather than use something that "Blocks" sites like netnanny, I would recommend using a "scripting" or "logging" program. We use linux at the house, so it is a geek thing (I had to write the script myself), but there are lots of windoze thingies out there that record keystrokes. I let my girls know that they could surf anonymously, but if there was any evidence of talking inappropriately or hanging out in rooms where that went on, then we would have to re-think the privileges. I pulled out a session in a chat room, printed it off, and told them both when it occurred (just to show them how it worked). We had no real problems with the internet, just knowing that it is not "private."
I have some friends who have struggled with internet porn, and we have a group in church that simply mail each other the log files once a month. We have a little widget that 1) tracks the installs of all new programs (a guy who thinks "just this time" can install another browswer and then delete it to "remove the evidence" 2)sends out a list of the files in IE or Firefox or Opera. It is good for accountability.
I know about teens and chat rooms, but in my opinion, adult men and porn is a far worse problem. All that assumes men are professing Christians. I doubt non-professors would see much problem with it.