Step 1: Click My Webinars on the left menu.
Step 2: On the next page, look for the webinar which the unwanted registrant is registered for. For live webinars, click Edit to the right of it then click View Statistics from the dropdown.
For automated and webinar series, click Show All then click the View Statistics icon to the right of the individual webinar.
Step 3: Scroll to Registrants and Attendees. Here, you'll see all the email addresses of those registered to attend your webinar.
Step 4: Click the Unregister icon to the right of the email address you want to remove.