Always a possibility I suppose
The guardian rock(s) could be under UN control. I assume that even the UN would favor saving the world from an NEO.
If our rock were overtaken by Dr. Silvana, PETA, Greens or some other evil force, there would hopefully be sufficient lag time to send up the space Marines before it could be launced at Earth.
My main wonder would be whether or not a major neo could be sufficiently destroyed by collision to eliminate the threat. The popular opinion seems to be that diverting the neo is the only safe solution