Absolutely trivial for an interstellar space-faring civilization to wipe out an up-and-coming species. Grab a medium size asteroid[1] from their solar system, adjust its trajectory slightly, bam! An extinction event level weapon. This works because your basically rolling the rocks downhill, and there is a tremendous potential energy difference[2] between Earth and say, the Kuiper belt[3].
Or a bit of biological warfare - can't be too hard for a super-advanced civilization to tweak a virus to be a little more dangerous. Or maybe a bit of radioactive dust, or just poking a whole in the planets ozone layer; there are probably lots of subtle techniques that an advanced AI could pull off too. the point is, is you command the technology and energy reserves needed for interstellar travel, you don't really need to fight a prolonged war against a species on a lower technology level. Just check in every few thousand years and if it seems like they're starting to expand just wipe them out.
[1]: https://en.wikipedia.org/wiki/Asteroid_capture
[3]: https://solarsystem.nasa.gov/solar-system/kuiper-belt/overvi...