Definitely soak it with a penetrating oil for a while, maybe even spray a good amount down the seat tube, then work on it over the course of a day or so.

If it still doesn't budge, I've had luck with a punch and hammer on stuck adjustable cups, but it'll do more damage than your spanner wrench. When you turn the spindle, is it very rough? If so, the bearing surfaces are probably shot anyway, making destructive removal an easier choice for you.]

or

Remove the fixed cup first, as miamjim's thread instructs, then use the Sheldon Brown tool on the adjustable cup: http://sheldonbrown.com/tooltips/bbcups.html