Gather the following items-
1) PB Blaster or other penetrating oil (WD-40 is alright, but doesn't seem to penetrate that well).
2) Propane or MAPP gas torch
3) Vise Grips (mid size works best)
4) Spoke Wrench or Adjustable Wrench (spoke wrench is better, but the latter will work)
Apply PB Blaster to all the siezed nipples, let it soak for a night or so, then try to hold onto the spoke with the vise grips and try to loosen the nipple. If this doesn't work, you might want to remove the tire and tube, and then heat up the nipples w/ the torch, and then break them loose w/ the wrench and "grips". The reason I say to remove the tire and tube is because the nipple can heat up and pop the tube possibly. Good luck. I dealt with this situation on almost all the spokes on my rear wheel- it was no fun, but a little sweat and blood (knuckles :bang: ) and they all gave up.