Can you not stop it by sending it commands to turn off the reactor?
Then it’ll crash and you’ll have a nice Chernobyl to deal with.
Several orders of magnitude less than Chernobyl. The reactor that lost containment was a gigawatt production plant. A gigawatt nuclear ramjet would be so huge there would be no trouble detecting it and shooting it down near the launch site.
Most of Chernobyl’s material stayed in the reactor, though. A hypersonic crash would disperse a much bigger proportion of the material. I don’t know how the two factors balance out exactly.
Just crash it into the ocean, sea water is an excellent neutron moderator.