Perfect, thanks. So if you just want to get the rows where the checkbox is not true, you would add an XPath filter to your tableRows selector.
Let's pretend the field bound to the checkbox is field3, ok? You could also check that the field = false instead of != true, but I guess I'm assuming any blanks should also be deleted:
XPathNodeIterator tableRows = domNav.Select("/my:myFields/my:Invoices_List/my:group6[my:field3 != 'true']", NamespaceManager);