If the organizer's account was removed, one way is to restore this account and use this account to cancel the meeting normally.
However, if it has been removed for longer than 30 days, you may not be able to restore it.
And you may need to remove the meetings from the attendees' calendar as Andy said.
To do it on the server side, you may connect to EXO powershell and use the Search-Mailbox command.
The command would be like:
Search-Mailbox -identity <attendees' mailbox> -SearchQuery "kind:meetings AND From: user@contoso.com AND subject:test" -DeleteContent
It would search all users' mailboxes and delete the meetings from user@Company portal .com which have the subject named "test".
Before running this command to delete the meeting directly, it is also recommended to first generate a report to see if the search result is correct:
Search-Mailbox -identity <attendees' mailbox> -SearchQuery "kind:meetings AND From: user@contoso.com AND subject:test" -TargetMailbox admin -TargetFolder inbox -logonly -loglevel full
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