To Ignore Another User's DM's: Go to the profile page of the user in question and click the action menu icon (three dots in top right corner) and select "Ignore User". You will no longer receive any DM's from this user, however you will still see messages from that user in public groups you both are subscribed to. When you are ignoring another user, that user is not notified, when they send DM's to you, they will just not get sent to you.
To Stop Ignoring Another User's Dm's: Go to the profile page of the user in question and click the action menu icon (three dots in top right corner) and select "Stop Ignoring User".