import bpy
from bpy.props import BoolProperty

bpy.types.Object.protected = BoolProperty(name = 'protected', default = False)
def main(context):
    for obj in context.selected_objects:
        if not obj.protected :
            bpy.context.scene.objects.unlink(obj)
            bpy.data.objects.remove(obj)
        else :
            print(obj.name +' is protected')


class delete_override(bpy.types.Operator):
    """Tooltip"""
    bl_idname = "object.delete"
    bl_label = "Simple Object Operator"

    @classmethod
    def poll(cls, context):
        return context.active_object is not None

    def execute(self, context):
        main(context)
        return {'FINISHED'}


def register():
    bpy.utils.register_class(delete_override)

def unregister():
    bpy.utils.unregister_class(delete_override)

if __name__ == "__main__":
    register()