Google+ sends you spam even you are not part of it. If you try to click on unsubscribe link, it leads you to sign up form to Google+. That is insane. The only defense is to report it as spam and use this:
from:(@plus.google.com)
subject:((invited you to join Google+) OR (do svých kruhů a zve vás na Google+) OR (příspěvky na Google+) OR (sharing with you on Google+))
Skip Inbox, Delete it