First, the letter/demand was returned with the blank Form W-9 enclosed. Second, a three-page letter was written, explaining the illegalities of the purported "requirement" and its federal penalties! Third, upon receipt of the would-be victim's letter, the Accounts Payable Dept. wrote: "the corporate office has processed your invoice. We regret any inconvenience . . ." Success!
More specifically; here's essentially what was involved: The IRS's Form W-9 is captioned as follows:

The letter stated:
Exhibit A and A-1 of C.U.R.E.'s Complete Withholding Exemption Certificate, consists of such SSN information.