There are two options to block a member, either a soft block or a full block.
A soft block is where you block a member who is on your friends list, so that they remain your friend, but they cannot see you online and you do not get updates in your friends activity. To soft block a friend, go to your friends settings page and hover over their name and choose the Block option. Your friend will now appear in your friends list and your blocked list.
You can remove this soft block on a friend by hovering over their name in the blocked list and choose the Remove Block option. They will then become a full friend again.
To block a member fully, you need to firstly, remove them as a friend. Then you can choose the Block option available on the context menu when you hover over their name or you can enter their username on your friends settings page. Once a member is blocked, they cannot see you online or contact you in anyway. You can remove a blocked member from your settings page. Simple hover over their name and choose the Remove Block option.