How does a user invite themselves onto my friends list. Previous contact with them was through spam mail.?

I received some spam mail some weeks ago and now the same user, from mac.com, has now invited themselves onto my friends list. What can be done to stop this kind of worrying intrusion.
How does a user invite themselves onto my friends list. Previous contact with them was through spam mail.?
every mail have a unsubscribe option you may try to unsubscribe to these types of mails.