[maratd]

Unless you assume each solar system is a threat, so you proactively send automated machines to each solar system that can adapt to the threat as needed. They might also not care if there is or isn't life there and just want to build Dyson spheres everywhere.

[ncmncm]

The argument for Dyson spheres is extremely weak. If you are powerful enough to build one, you are also powerful enough to dismantle a nearby gas giant and fuse that.

[DaiPlusPlus]

The energy required to dismantle and fuse a gas giant would be tremendous - there's definitely an opportunity-cost involved.

Compared to a Dyson sphere - which only requires a tiny, tiny, tiny fraction of the energy absorbed to build and launch the orbiting platforms that make-up the sphere.

You can "ignite" a gas-giant by maneuvering a black-hole into position inside the gas-giant, but (as far as we know) there are far more gas-giants than there are black-holes, and the distances involved are tremendous (nearest black hole: 3,000ly, nearest gas-giant: 6 to 9e-5 light-years). It just doesn't strike me as feasible or even worthwhile even if it was feasible.

Maybe as the heat-death of the universe approaches, the last band of remaining humans (don't ask) try to ignite the gas-giants to keep the lights on?