The Kindly Ones by Melissa Scott

1987 Baen book club hardcover edition in good shape. 
 
Orestes was a cruel world, cold and inhospitable. Its first colonists were castaways from a crash landing, clinging to survival through the institution of strict socio-political controls. Over the generations life grew somewhat easier, but the code of honor remained. Misdeeds, and errors, were paid for with blood.

At one time all miscreants were executed. Now, a social death is imposed. Every Oreseian city has a colony of "ghosts": ostracized citizens who must survive, somehow, without help from the living.
But galactic civilization is spreading - and Orestes is in its path. The old ways are under scrutiny. And though the Oresteian aristocracy will fight for the status quo, they have not reckoned on the power of a thousands ghosts...