I would send a huge asteroid hurtling toward their planet. I'd nudge it out of it's orbit by putting jets on it that ignited in the right angles and at the right times to put it on a path towards where their planet would be when my asteroid got there. I'd pick such a hugh asteroid that they'd not be able to blow it up or push it out of line. I'd send a squadron of fighters along with it to make sure no one tried to land on it and mess it's trajectory up. When that asteroid hit their planet, it would wipe them all out but not be big enough to destroy the whole planet. Then when the dust settled, I could terraform it and populate it with the people loyal to my rule. Or if it is really rich in minerals, just mine the heck out of it until all the minerals are extracted. Then maybe put prisoners on it as a prison planet and leave it in a wrecked conditon