We are talking about megastructures taking millenia to be build, requiring more resources than the ones that can be found in a single solar system, and that pro

We are talking about megastructures taking millenia to be build, requiring more resources than the ones that can be found in a single solar system, and that probably requires materials that may not be possible at all. And they are an answer for a problem that we, with our limited knowledge, can think of. They may be the wrong answer for civilizations that should be far more advanced than us to be able try to do something like that. But taking an idea similar to the Three Body Problem one. What your civilization should do if within a reasonable distance you find a Dyson sphere already built or in process of building, specially if it is far outside your technological capabilities, and knowing that if they require a Dyson Sphere then that civilization is actively expansive? Doing something to harm that may be a reasonable course of action, and it may not require very advanced technology (like sending an accelerating device that don't need to slow down).