I might have a better idea. There used to be a group which put out shock sites. I think their name was "Last Measure" or something of the like. Anyway one popular tactic they used was to trick people into visiting their sites where it would display images like ******* and have a voice announce something really loud such as "HEY EVERYBODY! I'M LOOKING AT GAY PORNO!!" Here's a Youtube video which kind of shows it
https://youtube.com/watch?v=Kui6cCu6tfA
So this might be a good tactic too. Just try to make it as loud as possible at their current volume. You could also be sneaky and make the volume for the first 30 seconds really low in order to trick them into turning it up higher...then hit them.

The idea being to embarrass them and get them pissed at the site hosting the videos. If this is too strong for you maybe funny but guilt inducing messages might also be effective. "DOES YOUR WIFE KNOW YOU ARE ON THIS SITE?", "HOPE YOU COVERED YOUR TRACKS WELL ENOUGH IN YOUR BROWSER HISTORY", "WHAT WOULD YOUR PREACHER SAY???" ;)
Another trick which might be worth a shot could be to scare them. Put up a blurb 30 seconds into the video, ATTENTION! ATTENTION! YOU ARE WATCHING AN ILLEGAL COPY OF A COPYRIGHTED VIDEO AND YOU MAY BE COMMITTING A FELONY. CURRENTLY LAW ENFORCEMENT AND COMPANY INTELLECTUAL PROPERTY DEFENSE ATTORNEYS ARE CONDUCTING STINGS TO IDENTIFY AND PUNISH VIOLATORS SUCH AS YOURSELF. PLEASE CLOSE THIS VIDEO AND LEAVE THIS SITE IMMEDIATELY OR RISK PROSECUTION!"
The average person is dumb enough where it will work.