You can only block another member if they've sent you a message.
Go to Messages and select the message thread. At the top of the thread, click on the user you would like to block and select "Block" from the menu.
This will not alert the person to the fact that you’ve blocked them, but will appear as though you are no longer accepting messages.
You are not able to block a member from tracking you. If your profile is public, all updates and activity will still be seen.