Okay, then parallel projection is what you're looking for. I tried it myself actually, and noticed that it won't work if the camera is already set in perspective mode.
Here's some code that sets correct parallel projection:
cam.setParallelProjection(true);
float aspect = (float) cam.getWidth() / cam.getHeight();
float size = 1f;
cam.setFrustum(-1000, 1000, -aspect * size, aspect * size, size, -size);
The "size" variable decides the size of the frustum, larger frustum size means objects will appear smaller on the frustum, while smaller frustum means objects will appear larger.
I also added a test to demonstrate capability of parallel projection.