How can I delete a Company from QuickBooks Desktop?

A QuickBooks company is just a file (really a small collection of files, but the *.QBW file is the only file containing actual financial data.)

You can delete it like any other file, like a spreadsheet or document or text file, using Windows File Explorer.

However, note that there is no real need to delete the file: You can create and use as many company files as you want with QB Desktop. There's no additional charge when you do so.