Any way to remove/reduce it with software?
With long exposures (more that a couple of seconds) your camera should automatically take a second picture after every exposure with the shutter closed. It then automatically subtracts this photo from the first one. This is called "dark frame subtraction". As this essentially doubles the time to take each exposure, CHDK allows you to turn this off (on the assumption you will post process the darkframe out). To do this, you need to periodically cover the lens and take a shot of darkness.
The menu option in the RAW menu called "Dark Frame Subtraction" determines whether the camera uses the internal process or not. Its active regardless of whether you have RAW enabled or not.
If its set to On or Auto you should see a reduction in amp glow in you pictures.