1. To Block a contact, all you have to do is right to left swipe* on the selected contact 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.