Wow. I certainly know how you feel. I've faced that many times. My parents I could almost say despise the internet. They say it's not healthy, I'm obsessed, it's dangerous, well, so many things. But yes, I know what you mean, about how one misses one's internet friends, messageboards, iming, everything. If my parents ever found out half the stuff messageboards I visit, or how I use it as a source of information on interesting topics, I'd be dead.
I couldn't write a letter or anything because my handwriting's HORRIBLE, but maybe they could just take away like the internet access?
Other than that, try to give them a hard time. That's what I did with mine. I would simply dress in my worst clothes, spend time running after and playing with the cat, listening to my music, the same song over and over and over again, which they hate, although now I know why I like it, reading my books, and watching my cartoons, and doing everything they hate to see me doing, which is just about everything about me. The point is, if they gave me my laptop back, they'd be free of me.