How can I remove the stem on my leaking IKEA faucet?

I believe that part is factory assembled, not removable. You may want to check with the manufacturer, if there's a local 800 number. Maybe they'll give you a brand new one.

While you have removed the locating screw, there are usually 3 rubber (well nitrile or somesuch) sealing rings which are a tight fit.

So, you might have to twist and pull very hard to get them to release.

At least, that was the case when I took a similar tap apart.

Of course, lots of care needed to get it back together without damaging the new seals...