first you can either send a polite email or get right down to sending them a c&d. if they ignore it - and i'm sure they will - you contact their host. if that doesn't work, you could talk to a lawyer.

keep in mind that a lot of the guys who are into file sharing aren't likely to become paying members, and some might actually type in the url on one of the video if he likes what he sees enough and is open to buying a membership.