For individual threads you are subscribed to you can still do a whole batch of them with just a couple clicks
First, go to "Settings" and click the "show all subscribed threads" button at the bottom of your list of recently active subscribed threads. On that next page, there is a checkbox at the far right for each thread listed. Just check the ones you want to delete and use the button below the subscribed threads list (on the bottom right) and there is an option to "delete subscriptions"
I think you'll be able to figure it out from that, but if you want screenshots just say so and I'll snap a couple