How to delete a material applied to my model and bring it back to default?

with the material selected first you need to press the delele key on your keyboard... if that doesn't work then another workflow is to select the entire model, give it all a random material from the assets list and export the model as Obj

inport the model you just exported back in,, delete the first model, and then select the material of your new inported, press delete to remove the materal... you should now have the default blank white