Every call that you receive will appear in your call log.  Calls that are completed successfully will have a block block symbol 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 redred block symbol.  (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.