You silly people! Just go into your user control panel and then, in the Edit Options area, under Messaging & Notification, select whether you want to receive daily emails, weekly emails, or no emails at all, for threads you're subscribed to.
By default you're subscribed to all threads you post in. But you can turn that off, so that you'll only be subscribed to threads you specifically click the "subscribe" link for.
Then you can keep email subscriptions on, but it will only be for threads you specifically opted into receiving email updates on.
Without knowing the type of emails you are getting, it is difficult to pinpoint the cause. But usually you get so much emails because of the notifications that you get when someone replies to a thread that you replied.
You can disable it by setting the "Default Thread Subscription Mode" in this link to "Do not subscribe" or "No email notification"
What, exactly, are the emails you're getting? Because it might be that you're not subscribed to individual threads (getting emails when others post in them) but subscribed to entire forums, receiving notifications when new threads are posted?
Gah! I was wrong! It looks as if each thread you're subscribed to can be on a different notification plan ... instant email notification for one and no email notification for another. So jbennett is correct in that changing the setting only applies to future threads.