The EPA probably can't directly, but the US government has full control over what crosses its borders. If the appropriate government arms decide VW is not allowed to export these cars from the United States intact, that will effectively prevent VW from reselling them elsewhere.
VW won't be doing a buyback because they simply decided on their own to do so. It will be part of a deal with the US to settle the case. If the terms include a resale prohibition, VW will have to honor that if they want to keep the case settled.