I noticed today that when you reply or create a new forum post the defaults are set to subscribe to the post.
I don't want to do that, so I set my profile to not subscribe:
Click on My Profile
Click on Edit Account Details:
Select which Subscription Updates you would like.
I chose Don't Subscribe, but you have the option to receive:
Individual Immediate E-mails (every response to any post you reply to or create gets sent immediately)
Combined Digest - Daily (a daily e-mail of all responses to any post you reply to or create)
Combined Digest - Weekly (same as above, but weekly)
No e-mail (the posts will show up on your home page for the forum)
Automatic (no idea what this does)
Click save and you're good to go.