Inbox full, how to unsubscribe from email newsletters?

(1) Use the unsubscribe link if an email has it; (2) block the email address in your email program. Probably they will respect (1), but maybe they won't, so always use the 1-2 punch.

If is it utter spam (girls in your area want to meet, V1agra, prOn), block the sender and optionally report spam. If it is something that you likely signed up for, click the unsubscribe link. If it is something you did not ask for or ever sign up for, click spam. You can also set filters to auto delete or put into folders to keep the inbox clean.

Then (3) you could report them to the Spam Cop! https://www.spamcop.net/