I spent a summer (or it felt like it) removing 3 aluminium pins from 3 steel frames. I had been a bit lax about maintenace up to that point.
They all came out. I tried most things. I can't say that one thing did the trick on any of them. I used ammonia (toilet cleaner from Wilko, I think), leverage (hole through the seatpin, then a long steel bar as a lever) and, on the last one, the cutting method.
Cutting the pin isn't as difficult as it sounds, if you give yourself enough time.
Allow 30 mins (lucky) or 1-2 hours (unlucky). Take it easy.
Cut the pin off about 1.5" above the seat tube. Get a good hacksaw blade. Wrap one end with some padding.
Lucky:
Cut down the visible pin and into it below the tube line, at an angle so that you aren't cutting the steel tube. When you've cut some pin inside the tube, try crushing the available tube with big pliers or a G cramp. If you're lucky (see subheading) you'll free enough of the stuck pin to be able to move the rest. Keep using lublicant/wd40. If you get any movement at all, the oil/wd40 will get in there and help a little.
Unlucky
Ok, so cutting the pin down from the top hasn't worked. It didn't work for me either. I just hoped it might. Now you're in for the long haul.
Start sawing so that the end of the hacksaw blade in the tube is touching the tube. The end has no teeth on it, so won't damage the inside of the tube.
Slowly, very slowly, over a period of minutes, maybe hours, possibly days, bring the blade more in line with the tube (but never exactly in line, because then you will be cutting the tube). Even if you do not cut the pin to the tube - something to avoid - you will weaken the internal structure of the pin. The idea is to do so until it is weak enough to crush it. Be prepared: you may need to cut it twice (two parallel cuts down the inside of the pin).
Eventually though it will give way. You just have to be really careful and very patient.
When you have done it, you will feel many things. I'll be grateful if you can put those feelings into words.
Paul.