Email the office of Attorney General of your state with a grievance - you paid for it, it was yanked from you, no one is picking up the phone. This is what AG is for - protecting consumers from all kinds of snatch-and-run, big and small.
Now you might think the AG won't have time for you, but the AG is not sitting there reading the incoming email and deciding to act. The AG clerk on duty will check that basic facts and dates are present in your email (make sure to include them), and ask the BigCo for their side of the story; all that before anyone even looks at the merit of the case. The BigCo will now face a choice - continue corresponding with the office of AG (which is billable lawyer time plus a drain on management brainpower), or shut you up by giving you back your stuff (which is free).
Now you might think the AG won't have time for you, but the AG is not sitting there reading the incoming email and deciding to act. The AG clerk on duty will check that basic facts and dates are present in your email (make sure to include them), and ask the BigCo for their side of the story; all that before anyone even looks at the merit of the case. The BigCo will now face a choice - continue corresponding with the office of AG (which is billable lawyer time plus a drain on management brainpower), or shut you up by giving you back your stuff (which is free).