You can block another member of Daily Challenge by clicking on their profile picture, then clicking "Block User" under their picture on their profile page. After blocking someone, you will no longer see any of their posts or comments, nor will they be able to send you private messages.
If you'd like to unblock a member you previously blocked, visit your Blocked Member list here. On this page, click "unblock" next to the member you'd like to unblock. Please note that you will not be automatically reconnected with a member after removing them from your block list, rather you'll need to request to connect with them once again.