You probably accidentally hit the subscribe to topic button. When you get the next email for the offending topic, there should be an unsubscribe button. Click it and the emails will cease.
When you select the reply button, check to see if there is a checkmark in the box to the right of the word "subscribe" at the bottom of the reply form. If so I would suggest that your default is to have this checked. I'm not sure how to alter that default, but in the meantime, just start unchecking it when you post and send ICQ a PM about it. Maybe he can sort out your default setting. If you learn of a way to alter the default setting, let us know because I bet there are a few other posters with the same issue.