The world government has been chasing Nico Robin for reading history of Void Century on Poneglyph stones. But why won't they just destroy it so the problem is solved?
According to wikia, Poneglyph is indestructible.
The blocks have no real power themselves, apart from their indestructibility; it has been stated that not even explosives could leave a scratch on these stones. However, the words they contain have the power to change the world.
Each one tells a piece of history long forgotten. This history includes mentions of (at least) three weapons of mass destruction: Pluton, Poseidon and Uranus. There are two types of poneglyphs: those that carry information about how to reach the other poneglyphs and those that carry the records of the "True History". Only when the stones are read as one will they fill in the Void Century of the world. In the eyes of the World Government, they are dangerous artifacts and despite declaring the stones dangerous because of the weapons, the reality is that the ideals of the fallen Kingdom written on some of the stones are far more dangerous then any of the weapons.
Because the Government stated that Poneglyph are dangerous artifacts and the blocks also indestructible, the Government prefer to kill peoples in Ohara rather than destroy the Poneglyph.