Basically, Adam Strange and everyone living on the planet, using their tech which includes zeta beam technology, capable of teleporting entire planets or suns..
That tech let them beat Onimar Synn the second time.
And if push comes to shove, all they have to do is keep the planet from Galactus.. Which they can do by teleporting it anywhere in the universe.
The Omega beamer could also convert the entire universe to energy, in the blink of an eye.
Not sure how it would affect Galactus, who's already energy inhaiting an M-body, but maybe Big G would spare Rann in exchange for one of those suckers, than he could just dump the heralds and teleport in all the planets he wants, in energy form and ready to absorb.
Alternatively, Rann could threaten to make the Omega Beam open source, effectively ending his threat to populated worlds and depriving him of sustenance.