In general, you will probably want to keep records on previous members of your program for reporting purposes, therefore when companies leave we need to set their status accordingly.
On the Edit Company page:
If dates are being captured, you should enter a valid date in Date Left
The priority should be set to Historic
The Company Type should be set to an appropriate status such as Historic
The enabled field should be left active - when a company is disabled, this simply removes them from being counted in reports.
Once these values are set, you should be able to submit the form and commit these changes.