How can I cleanup Company accounts data (dat)

I imported some demo data to the default Company(dat) accidentally and now I want to clean up the data for this company. I tried to delete this company and re-create it but it stores system data so the action is prohibited. What should I do ?

It wou;d be better if you create a new company and import useful or demo data into that and work with that. Because cleaning up can be done by selectively deleting record which would be a time consuming(unnecessary as well) task. Regards, Anuj