Add this to your /Editor folder in Unity to avoid importing materials for meshes.

DontImportMaterials.cs

using UnityEditor;

public class DontImportMaterials : AssetPostprocessor
{
	public void OnPreprocessModel()
	{
		ModelImporter modelImporter = (ModelImporter) assetImporter;                    
		modelImporter.importMaterials = false;
	}  
}