In your Notifications settings, (Profile->Account settings, Modify Profile->Notifications)
For topics and boards I've requested notification on, notify me of: Nothing at all
I just tested it and it looks like it's working (which is to say, not sending the email)
This does disable notifications but then I'm not sure what else selecting "Notify" on a thread would do, since there does not appear to be a place to access subscriptions.
The closest thing I can find to a subscriptions page is the link up top to "Show new replies to your posts" but using that has several problems as far as I can tell:
1. there does not appear to be a way to stop seeing replies to threads you're no longer interested in
2. threads you have not posted in will not show up
3. after checking the newest posts on a thread, it will not provide a way back to it until someone else posts