Also, the RC infection may not be so much an enchantment as an alteration in the nature of the being. And even if it was an enchantment, it's a full-body, internalized one- breaking it with a warden's sword would be like trying to surgically destroy every virus in the body to cure a disease.
As for the pentagram, Ms Duck has a pretty plausible explanation. If the enchantment is violently released, slashing blindly through something like that would be somewhat suicidal. Actually, I don't remember off the top of my head, but I don't think Luccio was even there for the climactic battle of SmF. It was just Harry and the Knights on Demonreach, with the rest as backup.