Please note that this section is for questions regarding the forum itself - it is not for fan game-related questions. If you have a question about a fan game, ask in the appropriate thread.
Welcome to PokéCommunity! Register now and join one of the best fan communities on the 'net to talk Pokémon and more! We are not affiliated with The Pokémon Company or Nintendo.
In the Options page of your UserCP, you will find a field called "Default Thread Subscription Mode".
Set the box there to "Do not subscribe".
That should fix it.
ALso, when replying to a thread, the "Thread Subscription" option in the "Additional Options" table, below your post, needs to be set to "Do not subscribe". (Usually this is set through your UserCP.)
Do you use Quick Reply? If you do, you can't stop it. You might, though, check User CP --> Options. If you use normal Post Reply, there's a box that says "Subscribe to this thread?", un-check it.
[EDIT] Haha, Scizz beat me to it.