In the Blocking section of the admin, you will see a section for blocking members. Search and click the member name and they will be blocked.
Blocking non-members can be done by blacklisting their IP address. In the Blocking section of the admin, you will see an IP black list section. Click Add IP, then add the user's IP address:
Please note that this cannot be 100% effective as the person could contact you via another IP address.