I have an i-phone, but sure it must be similar on all phones. In the recent phone history where you see the number displayed look to the right. There is an "i" there on the i-phone, but it's a shortcut to add a contact, so wherever that is for your phone. It pops up so you can add contact info, but if you scroll to the bottom of the page there is a button to block contact. Click it. It will tell you contact is blocked, press button to unblock, blah blah. Then just exit out of that screen and you're good to go.