Sure, it's a reason to blackhole the isp, but this is the Internet we're talking about. So say its upstream providers will backhole it, say even the tier 1 providers do the same.

That won't stop it from connecting to smaller networks and announcing the same fake routes there. And even if the small networks blackhole it, the attackers can just register a new AS.

Also, there are cases in which attackers are performed from large networks, with millions of users. Are you going to blackhole such a network? They'll apologize and claim that it was a configuration error.

Note that these types of attacks are generally not performed by amateurs. They're performed by states, intelligence agencies, criminal organizations and the like.