I use carboard boxes, willow chewsticks (from west wales willow), cholla and lots of bits of natural straw mats or woven baskets (from chinchillas2home) to distract mine and encourage them to chew those instead. Whenever they chewed the wall I shouted stop and clapped my hands then offered them a toy. This seems to have worked and my two have been pretty good recently *touch wood* !
Caz