There is a way to do this but iFixit doesn't have a specific guide for it. Instead use this guide
[guide|7037]
to remove everything up to the fan. Once the fan is removed then unscrew the five screws in the white circle at the top. Once these are unscrewed you can pull the arm off slightly and if you are never going to try and use a display again then cut the 5 wires leading to it. If you want to use the display again then you can reroute the 5 cables and pull them out.
Hope this helps and you can decapitate you iMac.
Here is a link showing pics of what one looks like.
http://www.flickr.com/photos/willceau/4313598241/in/photostream/
http://www.flickr.com/photos/willceau/4323288972/in/photostream/