Use loader.loadModelCopy() instead of loader.loadModel() if you are loading multiple copies of the same model. This method will try to load from the cache first, which is faster than loading it from the disk every time.
If you want to remove a node from the scenegraph, you can just use self.obstacle.detachNode() for the obstacle you want to remove. See section A in programming with panda in the manual for basic scenegraph manipulations.