In Poser 5 the only way to cancel a render is CTRL ALT DEL and close Poser.  

In later versions IE: Poser 6, cancel does work. However, the time it takes to cancel depends on your bucket size.  Before the render will cancel, Poser will finish the current bucket that it's working on. If you have a high number for your bucket size, then Poser will continue to process that bucket amount until it's done, and then cancel the render.  If you have a smaller bucket size, the time to cancel the render will be less, but obviously the time to render the image will be longer.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi