I've turned off all alerts, I've turned off auto-following threads, I have saved these preferences - but nothing works. I keep getting email alerts every god damned time someone post in a thread I have also posted in. How the hell do I turn this off?
Hover over your username in the navigation bar, click "Watched Threads". Click "Show all watched threads". Click all the check boxes. At the bottom of the list is a dropdown box that says "With selected...", choose "Disable email alerts" and click "Go". Go to next page of watched threads and repeat steps 3-5. Not the easiest thing in the world, but should only need to be done once.
When you turn off the alerts, what it actually does is turns off auto-subscription to alerts for threads you post in in the future. It doesn't cancel existing subscriptions, and so you'll continue to get email notifications for threads you are already subscribed to. Kalin has described what you need to do to cancel existing subscriptions.