You can block a contact to prevent the caller from reaching your phone number. If a blocked caller tries to reach you, they will hear a "no longer in service" message. 

  1. Tap Contacts in the lower toolbar.
  2. Select the desired contact.
  3. Tap the Settings icon (three vertical dots).
  4. Tap Block Calls.

You can see a list of all blocked contacts under Settings > Blocked Numbers