Oh, please don't do this. There are so many legit businesses people opted-into who are penalized by this practice. The spam button is not a delete button. Marking things as spam HARMs senders. Click Unsubscribe as emails come in and you won't punish hard-working businesses trying to make ends meet.
Another tip - export all your mail and store it on a drive. You really don't have to go through it all - and 15GB is not much space.