FAR 33.206(a) gives contractors six years to submit a claim after accrual of the claim (i.e., contract performance). Thus, since the contractor has not requested payment by the end of year six, it cannot claim payment and the funds can be deobligated and the contract closed out. To demonstrate good faith you may want to send the company one more certified letter (after confirming the address), instructing it to invoice within 30 days of receipt. Document the contract file carefully. Also, consult with your activity's legal counsel for a more authoritative opinion on this legal issue.