You don't get more spam by marking something as spam. You get it out of your face and not annoying you any more. (and in the case of webmail services, you help other people not get spam in their inbox, by notifying your email service so they can automatically send it to the spam folder for other people)
In fact, the method you desire to use, the unsubscribing, is one of the surest way to get more spam, because you are notifying the spammers that it's a live email address with someone that reads and responds to spam. You become a valuable commodity, with your email address added to a list of special people to spam, that is sold at a premium price, to other spammers.
- Never visit a link in spam email
- Never contact the spammers
- Never try to unsubscribe from spam
- Never download or open attachments from spam.
Just mark it as spam and be done with it.