Paul,
Your user Adam C. most likely has a calendar subscription set up to import events from a google calendar. My guess is that the google calendar has a the recurring quiz event you found. (I found on my sites that when a user schedules a calendar import it is run as "admin" for that user and the logs fill up with one entry per event imported.)
You may want to try logging in as that user and changing the frequency of the calendar sync and see if that addresses your problem. Information about the calendar sync can be found here: https://docs.moodle.org/28/en/Calendar_import