How to prevent model from moving itself?

edit

I'm using urdf in gazebo to build a walking quadruped robot ,after set the proper inertial (calculated by meshlab followed the tutorials here link text), the model is still moving ,just floating on the ground,acts like someone is pushing it.Also when click my link in gazebo's left panel,some of the bounding box is really wrong. In addition,I just start gazebo without any controller,I found some useful anwsers: link text link text link text link text link text. But none of them fixed my problem.Right now I'm using ROS Indigo with Gazebo 2.x.Here is my urdf files divided into three parts:urdf.xacro, trasmission.xacro and gazebo.xacro.I'v been stuck in this problem for nearly a week.Can anyone help me fix this problem?