In this case, the process required to remove the company which had automatically been added was:
1. Delete the [default] Tasks in the [default] Project created by Odoo for the new company.
2. Delete the [default] Project.
3. Delete the company.
Contrary to what has been stated by Odoo support ("Once a company is created, we will not be able to delete it as dependencies are created in the system.") (sic!), the company was then deleted without errors.
What still remains an open question is how to disable the option to add new columns in a given Kanban view...