Every call that you receive will appear in your call log. Calls that are completed successfully will have a block 
symbol next to it. When you press the block symbol a confirmation box will appear, when you press "yes" the caller will no longer be able to call you and the block symbol will turn red
. (Note: when a caller is blocked they still have the ability to send you a text message.)
To unbock the caller simply press the red block symbol again.