Actually, I believe you can go to the thread you posted in, find one of your posts, click "modify," and then do nothing to the post, uncheck the "notify me of replies," and then save it. See if this works. You won't see the checkbox if you use the quick modify function (the little white icon that shows up on every post you've made).
Alternatively you can make a new post and uncheck the box from there, but if you aren't interested in the thread anymore you probably don't want to do that.
Edit: Yes, blubart is right, you can unsubscribe from there, I just checked.
