From your computer, copy your keys into the RPi:

$ ssh-copy-id -i ~/.ssh/id_rsa.pub pi@raspberry.local

To disable password login you need to update the SSHD configuration file:

$ sudo nano /etc/ssh/sshd_config

Search for the different lines containing this keys and set them to no:

ChallengeResponseAuthentication no
PasswordAuthentication no
UsePAM no

Reload SSH:

$ sudo /etc/init.d/ssh reload