There are some tools out there that allow you to hack Yahoo! Messenger, so long as you know the user name of the person who is using the program.
Anti-spy and similar programs can't stop it, because the program intercedes itself between your computer and Yahoo! in order to work.
How it works:
1. Jerk starts spy program and guides it to look for you.
2. Spy program polls server to find out if you are on.
3. If you are, spy program intercepts certain communications between Messenger and Yahoo!, to figure out what you are doing.
4. Spy program, via combination of eavesdropping, impersonating you, impersonating the server and / or the Messenger program of the person you are speaking / camming with, etc., allows Jerk to basically see and hear everything you do.
All this is possible because Messenger does not encrypt the data it sends back and forth, and was not written with client-to-server security in mind. Everything you do is sent in the clear, and can be intercepted; from there, it's easy to bypass your settings, etc., if you know how the program works, because everything -- your cam, your friends list, etc. -- are not secure.
Your best bet is to immediately change your chat user name. However, since he has your friends list, Jerk could just start eavesdropping on them, waiting for you to show up, and get your new ID.
The other alternative: What he is doing is illegal, period. It's called wiretapping and it's a federal crime in the US (unless you work for the NSA, but let's not get sidetracked). It's also a crime in England or any other Western nation, and even if it wasn't, it's still illegal in the US.
You should report him to the FBI. You can also contact Scotland Yard.
http://www.ic3.gov/
http://www.online.police.uk/english/default.asp