I’d imagine there could be an ethical way to do so through a sunset protocol similar to the concept of rapture (the religious kind, not the Bioshock city) - freeze simulation, move all the beings’ minds to “heaven”, shut down physical universe simulation (lowering operation costs by at least five orders of magnitude, I’d imagine), and let them enjoy afterlife until they get tired of existing, reach nirvana, or something like that.

That reminds me, I should really get back into AI research.