First log into your account. Click on your name in the top right corner (account) and select dashboard. For fast navigation to your dashboard, simply follow the link: https://www.entryninja.com/dashboard
On your dashboard navigate yourself to the registration of the event you would like to remove the member.
Only the registration owner can remove a member (the orginal person who created the entry).
Click on the red cross next to the person's name you would like to remove.