That's a variant of what I discuss in another reply, but well underscores my point of just how easy this is if you happen to get there anywhere in the 2.0-2.4 billion window Earth has had prior to intelligence. If you colonize the system in any manner, be it biological, technological, whatever, for Dark Forest purposes that's equivalent to destruction; there is no longer a threat of natural competing life.

You've got two major approaches: Fling a kinetic kill projectile, or if you have the tech to get "something" into the target system at roughly orbital velocities, send some machines to do the job with local resources.

None of these things appear to have happened in Earth's past. Ironically, trying to spin yarns in which they did anyhow still end up countering the Dark Forest hypothesis, because all such attempts must either include significant probabilities of failure of the attempts or the possibility that the life out there is benevolent (to some degree, at least sufficient to avoid simply wiping us out, which on this scale is "benevolence" despite whatever else they may be doing) which itself would imply the forest isn't that dark.