Not sure (never tested myself) but try to check windows event log for event 110:
110 Backup job started <Job type> job "<Job name>" has been started. Info Backup
In the event details look for the User field and associated windows account.
it defaults to targeting the system drive of the Veeam Backup server...
If possible, remove the default C:\Backup repository. Once done this location should not appear under repository thus users won’t be able to choose it.