I know it's an old post, but as deleting a company is not addressed anywhere and this was the first hit when I googled it, I figured I could add a suggestion here on how to delete a company.

As Dheeraj pointed out, to delete a record you must first delete all records in other tables that relate to it. As almost everything in Odoo relates to a company, it is probably going to be quite a tedious and time consuming job to do that through the UI. So, an easier approach to do this is through the database. 

1. Check which Id the company has by querying the companies table:

select * from res_company rc;

2. Execute the following update

update res_company rc set resource_calendar_id = null where id = 123; -- Replace 123 with the id of your company

3. Execute the following select to generate a bunch of delete statements to delete all data related to the company:

SELECT 'delete from ' || columns.table_name || ' where ' || columns.column_name || '= 123;' -- Replace 123 with the id of your company
FROM information_schema.columns
join information_schema.tables t on columns.table_catalog = t.table_catalog and columns.table_schema = t.table_schema and columns.table_name = t.table_name
where (column_name = 'res_company_id' or column_name = 'company_id' or (t.table_name = 'res_company' and column_name = 'id')) and t.table_type = 'BASE TABLE';

4. Now execute the delete statements.

Depending on your exact situation, you may get some constraint violations when executing the delete statements. You'll have to resolve those in the appropriate way:

• Reorder the statements to first delete items referencing another table

• Set foreign key fields to null in case of circular dependencies (step 2 is an example)

I also found that some tables do not reference res_company directly. For example, account_payment does not reference res_company. So you have to derive the company from a related table. For example:

delete from account_payment ap where journal_id in (select id from account_journal aj where company_id = 123);

In my database I also found records of company A referencing records of company B, but let's not dwell on that :)

It goes without saying that the above advice comes without any guarantee whatsoever and a big warning: BE CAREFULL

• Make sure you have a backup before you start

• Try on a test-database first

• Only do this when you understand what you are doing. Don't just execute these statements blindly.

If you end up with an empty production-database: too bad for you :)

(For some reason this text editor keeps screwing-up my text. I really struggled to get the queries above in, so it's possible some space or character ended up dropping off)

As per my knowledge,  you have to remove that company details from everywhere than only you can be able to delete the company. Let's say if a company is used in some accounting than you have to delete the accounting first and then delete the company

You can inherit 'res.company' model and add Boolean 'active' field , then in this company make active = False