I don't really understand how that works. After all I don't think a normal Internet can hijack BGP, they need to be able to announce routes. So if an ISP announces low-cost routes, you know who did it -- unless the ISP's security was breached quite severely.

So, can't you just asked the ISPs who announced those fake routes if they were hacked, and if not, demand an explanation? (And if no explanation comes forth, stop peering with them)