The Precursors are reality altering gods who swatted down entire species just for the hell of it. Their buildings, ships, and solar system-sized battle stations are all indestructible. The Flood has access to the same tech; and are also able to alter reality once they reach Keymind stage. They can infect ANYTHING, be it flesh, metal, or mind. Even the Force would probably be tapped into and corrupted. The Flood can not be stopped.