So. What if somebody take an old superdreadnought about 6.8 millions tonns in mass, get it out of hyperspace far enough of solar system to avoid detection and large concentration of space dust (some light-days should do the trick), accelerate it toward the system up to the 0,9 speed of light, then turn it impeller-forward and just dump it into the star?
The effect of collision would be... Interesting. The impeller would act like a dust screen, protecting the mass from solar wind, so the ship would be vaporised not before it stike the star core. I'm not completely sure what would happen exactly - after all, the stellar physics isn't the best understood direction of modern science - but something happen undoubtly. And if the calculation about the ammount of energy delivered to the core is right, then with good probablity we would have the neutronization process on the loose.
So, generally we would have sub-supernova scale torpedo.

Who need the deterrence weapon to hold this multi-system star nations out of your single, small star system?
