Hi
Go to the bottom of the posts and look for the Spanner symbol. Click on this and uncheck the top item - Subscribe/unsubscribe. It should not have a tick if you want to receive notifications for that topic.
Next ensure the 'Notify Me' option at the bellow a text box when replying to a post is checked.
![Very Happy :D](./images/smilies/icon_e_biggrin.gif)