You can delete subscriptions. Just go into the User CP and scroll down the listed options on the left until you find "List Subscriptions". Click on that and it will show you all of the threads that you are subscribed to. Once there check the box beside any threads you wish to unsubscribe from. Then below that is a drop down menu. The default wording says "Move to folder..." When you click on the little arrow beside it, you will see all of the options. Click on "Delete Subscription" (as seen below) then press the "Go" button.