You can check to see who has joined your webinar right before you start broadcasting, or during the webinar. You can see their names as they log in right in the webinar studio.
In the studio, there are two icons and a toggle underneath the webinar screen. The pencil icon is for registrants and the head icon is for attendees. The toggle next to the icons hides the numbers from the webinar page, so only the host sees the numbers.
Click on each icon and you will get a popup window showing a list of your registrants or attendees.
Clicking each name on the list brings up the option to Email or Block. Click Email and your mailing app will be opened in another tab or the associated app program. Click Block and the registrant or attendee will no longer be able to access or they will be kicked from your webinar.
You could always come back and unblock them.
You can always see who registered for and attended your webinars on your Statistics Page.