Delete appointment from a large list of user

Hi all, I am not really familiar with Exchange however we are facing a kind of issue 

the issue is  "user A" send and appointment to a large group

"User A" left the company but now all the user in the group still have the appointment on their calendars

is there a way for exchange administrator to delete that appointment on all the calendar?

at this point the only suggestion was to delete the series on each user calendar but as I say it is a Large group.

any comment?

thanks and Regards.