In order to block a number:
1) Go to Recents
2) Click the conversation with the number you would like to block
3) Click Gear icon > Block number
When you have picked one block option the number will be blocked for incoming calls and texts.
In order to see all the numbers you have blocked and unblock numbers:
1) Go to More > My Numbers > Click your number > Blocked #s
In this view you see a list of all the numbers you have blocked. A * means that all incoming calls and texts are blocked.
Click unblock to unblock a number.