You should be able to release such orphaned locks via impersonating the user first, as described at the end of this post: http://pholpar.wordpress.com/2014/04/03/how-to-use-powershell-to-delete-short-term-locks-from-documents-opened-from-sharepoint/

For example, using PowerShell:

$web = Get-SPWeb http://intranet.contoso.com
$list = $web.Lists["DocLib"] 
$item = $list.GetItemById(2) 
$file = $item.File 
$userId = $file.LockedByUser.ID 
$user = $web.AllUsers.GetByID($userId) 
$impSite= New-Object Microsoft.SharePoint.SPSite($web.Url, $user.UserToken); 
$impWeb = $impSite.OpenWeb(); 
$impList = $impWeb.Lists[$list.Title] 
$impItem = $impList.GetItemById($item.ID) 
$impFile = $impItem.File 
$impFile.ReleaseLock($impFile.LockId)

You should be able to release such orphaned locks via impersonating the user first, as described at the end of this post: http://pholpar.wordpress.com/2014/04/03/how-to-use-powershell-to-delete-short-term-locks-from-documents-opened-from-sharepoint/

For example, using PowerShell:

$web = Get-SPWeb http://intranet.contoso.com
$list = $web.Lists["DocLib"] 
$item = $list.GetItemById(2) 
$file = $item.File 
$userId = $file.LockedByUser.ID 
$user = $web.AllUsers.GetByID($userId) 
$impSite= New-Object Microsoft.SharePoint.SPSite($web.Url, $user.UserToken); 
$impWeb = $impSite.OpenWeb(); 
$impList = $impWeb.Lists[$list.Title] 
$impItem = $impList.GetItemById($item.ID) 
$impFile = $impItem.File 
$impFile.ReleaseLock($impFile.LockId)

After the lock has been released, you should be able to delete the document again.