with regards to the stem. Sounds like rust between the surfaces in the tube could be the problem. I had a similar problem and after having no luck with wd40, bought and used penetrative oil to get in there. With a bit more elbow grease i separated them. You should be able to get the fork off easily then.
If all fails and you don't care about your stem (ie if you're getting a new one) bash it out with a hammer.
with regards to the stem. Sounds like rust between the surfaces in the tube could be the problem. I had a similar problem and after having no luck with wd40, bought and used penetrative oil to get in there. With a bit more elbow grease i separated them. You should be able to get the fork off easily then.
If all fails and you don't care about your stem (ie if you're getting a new one) bash it out with a hammer.