You can delete them.
1. Click on Settings by Logout.
2. Click on Bookmarks under My Subscriptions.
3. Mark the bookmarks that you want to delete by clicking in the little box to the left of each bookmark. It will put the checkmark in the box.
4. When you have the bookmarks marked that you want to delete, click on Selected Threads under the list of bookmarked threads.
5. Choose Delete Subscriptions, then in the lower right hand corner of this box click on Go.
They should be deleted.