1. To block a contact, all you have to do is right-to-left swipe on the selected contact and the option to "Delete or Block" will appear.
2. To Unblock a contact, go to your "Settings" > "Security Settings," choose "Blocked Numbers" and then choose the number you would like to unblock. Right-to-left swipe over it to "Unblock" the number. If you are not on an Apple device please press and hold instead of swiping.