I just found out we have partially identified the source of this problem, and are currently working on a resolution.
As a workaround:
I apologize for the difficulty there.
This is currently a known issue in the behavior of the attendee join flow. It's pretty rare though, considering most folks will cancel well in advance of the live session.
We have the same issue but now when we cancel an individual we receive a message saying Failed to update registrants, Please try again ...
Tried from FF and Chrome same issue ...
Webinar has ended and a competitor registered for the recording.. We want to remove him so he won't receive the follow up email that has more documents
@SallyDuPont [UPDATE] We've flagged this issue on the new GoToWebinar interface for a fix. The messaging will improve on the site very soon.
Past webinars in the old and new vew should not allow the cancelling of registrants once the end date is over.