You can delete guests from your event in two ways.
One by one
Hover over the guest name in the list and look for the red Trash bin icon (see screenshot below). Hit the icon to delete the guest.
Bulk guest deletion
You can delete all guests in bulk during the import of the new list. Thus, replacing the existing guests.
To do that look for Advanced options during the import of the new list (Excel or CSV file). Select Delete the existing guests as on the screenshot below. Eventor.app will give you an indication how many guests will be deleted in the process.
During this process you may also set the guest status in bulk by selecting the appropriate status (Attending, Invited etc.).