You can't block someone, unfortunately but you can adjust your notification settings by adjusting your status updates. On your homepage/feed click on filters. Where it says filter by types click edit. You will see a list of check boxes, just tick the ones which apply to you. That should stop you from getting notified in your feed. If you stop following someone you should not be getting emails from their activities. Double check to make sure you are not still following them.
You can also adjust you email notifications by going to your account/following and adjust the settings for each item on the list. You may have commented on a hub of the person before you stop following but is till getting notified. In the hubs you follow section you will see a list of the hubs you follow just click unflollow for the ones you no longer which to follow.