To block specific newsfeed posts, or to block a member totally from your newsfeed:
First, you can click the 3 dots next to that any of that person's newsfeed posts.
After you do that, then you can select either to hide just that post, or hide all that user's posts:
To block someone from sending you Private Messages:
Click the 3 dots in the upper right of the message from the person you want to block.
Then, select 'block user':
To 'unblock' someone from your PM's or newsfeed posts:
*Make sure you click the 'Me' tab on the bottom, then click the 'gear' icon in the upper right for your settings:
Then, clicked the 'blocked users' section:
You'll then be taken to a page that gives you the option to 'unblock' members from your newsfeed or Private Messages. You can also add users here manually, if you already know the person's username:
Alternatively, to unblock Private Messages:
You can also 'unblock' a user from Private Messages by going into your last conversation, tapping the 3 dots in the upper right, then tapping 'unblock user'.