There is no way to unsub from everything at once as far as I know. But if you go to the workshop, click on browse and select "subscribed items" in the drop down menu you'll be taken to a list which has all the mods you're subscribed from. You can go through and easily unsub from that page

thank you

on the same page that Golly told you there is a big "unsubscribe from all mods" button on the right side, under your steam avatar