Can't see anything that says unfollow, not on any of the threads I'm tracking.
Just to clarify how I get to the threads: Front page>forum dropdown>(subcategory)>sign in>my profile name dropdown>tracked threads>double click on the item to view.
It's very strange because it seems to be totally random, none of them are marked as closed or solved and all can be found in the Forum pages, if you go far enough back.