Earlier quoted context omitted.
What about a shotgun approach? Take a comet, accelerate it up to a significant fraction of lightspeed (after all, you you have lightyears in which to do so), and trigger a bomb in the comet to blow it apart into a few smaller pieces, each traveling at a significant fraction of c . Rinse and repeat.
I've made another reply at the site: So the invading force goes into the Oort cloud, picks a chunky rock, attaches a giant thruster to it and begins accelerating it towards c. A day later, the speed of the rock has reached 0.5c. Hours later, a defending fleet arrives, shoots and kills all the invading b ds. The rock however, continues at 0.5c even though the thruster and guidance systems were destroyed. It reaches th…
Strap small N-bombs (or conventional bombs?) to as many as asteroids as possible, launch them at the target area, and then blow them up ~500,000km away. Suddenly you have thousands (millions?) of significant chunks of rock travelling at 0.5c and spreading out. If you got the explosion point right and had enough asteroids you could cover a reasonable amount of the likely positions, even based on random movements.
It's unlikely to destroy an entire asteroid-belt based civilization, but wouldn't be trivial to evade either.