Easy! Like any other social media: "@" + the name of your Keakr friend.
Example: "Yo MCKeakr, check my freestyle it's dope"
Everytime you mention a user, he receives a notification on his phone.
FYI, each abusive mention which can be assimilated to a spam will be deleted. In case of repeated abuse, your account can be suspended or even definitely removed.
Also: if you blocked a user, he won't be able to mention you of course.