While your main point stands, I doubt that crashing even a moderately sized planetoid into a star at any achievable speed would do much to that star. It definitely wouldn't cause a normal supernova.
You might be able to produce some nasty radiation/ionized crap/relativistic debris that would be unpleasant for that greenish scum clinging to some of the planets, but it'd be a really expensive way to do even that.
Sure, if you get close enough to C, you can deliver as much energy as you want, but you're probably going to use the energy of several stars to get a small object going fast enough to deposit a really destructive amount of energy into a single star.
At least as far as our current understanding of physics it is probably not possible to accelerate an object to the actual speed of light, but who knows if there is some kind of universe hack that we don't know of.
maybe not a planet but a mini blackhole tugged by a small ship at high velocity to crash into your sun and turn it into a proper black-hole with accretion disc and the works would do some serious damage to you as a species.
personally I'd go for something like a nanobot gray goo thing that infects & turns everything it touches in a given gravity well. then send over a bunch of ships to wipe out remaining stragglers.
You might be able to produce some nasty radiation/ionized crap/relativistic debris that would be unpleasant for that greenish scum clinging to some of the planets, but it'd be a really expensive way to do even that.
Sure, if you get close enough to C, you can deliver as much energy as you want, but you're probably going to use the energy of several stars to get a small object going fast enough to deposit a really destructive amount of energy into a single star.
Efficiency is part of finesse...