Thanks for all of the tips and advice. WD-40, acid soak, and the strap wrench were not enough to get the spacers removed. I ended up using the smallest chisel I had and light taps from a hammer to work through each of the four spacers. I put a vertical groove down each one, then turned the chisel 90 degrees so the back side was against the steerer tube and worked down. Not surprisingly, the last spacer was the thickest and most stuck.
The spacers were definitely stuck due to sweat / salt and maybe some light corrosion on the aluminum tube. I cleaned it up with emory paper. I’ll do some more when I take it all apart again to replace the upper and lower bearings. (I did all this to confirm the lower bearing that I needed.)
Now, how do I prevent this from happening again? Whenever riding inside I drape a thick wash cloth over the stem and head set. Should I spin the spacers before / after each workout and disassemble as soon as they start to stick? What else should I do?
Again, thanks for all of the help above!