In Book Keeper Android, click on three dots against company name and choose Delete option.
In Book Keeper iOS, Left Swipe on company name, Choose Delete from device option.
In Book Keeper Windows, first locate your company file.
Once you have found the company file, you can delete it by pressing Delete button on your keyboard.
If your company file is on Dropbox, go to Dropbox folder > Apps > Book Keeper App
There you will find the company file, you have to delete from there as well.