Multiplayer could fix that. We could stage a crusade to the nether. Arm ourselves. Some of us fight to keep them occupied and distracted, some of us build, until we have a stronghold worthy of note and drive back the ghasts, and forge for ourselves a great land of our own.
Just don't attack the pig zombies. They're not hostile until attacked, but they also happen to be armed with gold swords and pack a mean punch.
Also:
Ghasts can't see through glass, but their fireballs will destroy it.
Stone blocks can survive direct hits from ghasts.
The common nether rock is easily destroyed by ghasts.