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 you block someone, the two of you will no longer be connected. You will not be able to interact with one another in Daily Challenge. You won't see each other's posts or profiles, and you won't be able to send one another messages or invitations to connect.
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.