See this FAQ bottom section,
https://ttlc.intuit.com/questions/1899484-w-2s-and-excess-social-security-payroll-taxes-withheld If your employer does not (or cannot) comply with your request, use Form 843 Claim for Refund to get a refund of the overpayment.
http://www.irs.gov/pub/irs-pdf/f843.pdf and instructions
http://www.irs.gov/pub/irs-pdf/i843.pdf
