Or you could just smack it with a really big rock. That nicely sterlizes the planet too, so you can reseed it with life from your planet after to make it more comfortable.
Honestly it's all kind of silly because distances are so vast. It makes little sense to travel to a distant star - much better to build space habitats in your own star system. And even if you were going to do that for some crazy reason, there's no shortage of uninhabited planets out there. There's no need to commit planetary genocide of an alien race. Also if you have that kind of tech level, space habitats make more sense than planets anyway. Finally if you seed distant star systems with your own species, you risk that species going to war with you later (see how that played out on Earth just across the oceans.)
Honestly it's all kind of silly because distances are so vast. It makes little sense to travel to a distant star - much better to build space habitats in your own star system. And even if you were going to do that for some crazy reason, there's no shortage of uninhabited planets out there. There's no need to commit planetary genocide of an alien race. Also if you have that kind of tech level, space habitats make more sense than planets anyway. Finally if you seed distant star systems with your own species, you risk that species going to war with you later (see how that played out on Earth just across the oceans.)