First....DO NOT POUND ON THE KEYS DIRECTLY WITH A HARDENED HAMMER! It's very easy to "tweak" the keys, and if you do that, they will either never come out, or they will not be reusable.
OK, I'm not gona make any assumptions..... have you checked to see if somebody pulled a "stupid" and tack welded the key in place? Don't laugh, I've seen it a number of times. If you can ensure the key isn't tack welded..... My first move would be to spray the dickens out of the area with oven cleaner, and let it sit overnight. Then hose off the oven cleaner, THEN start soaking it down with the PB or WD-40, and repeat the next day, and the next, and the next. Each day, try to hammer the key out using a "soft" hammer.....brass, lead, etc. If you use a hardened hammer, place a piece of hardwood against the key before striking it with a hammer. BE PATIENT! I've overhauled a bunch of LG hammers, and getting in a hurry usually costs you big dollars due to damaged and/or broken parts caused by your own haste. IF, after several days of soaking it with PB or WD-40, heat might help...... the head of LGs is made of cast, while the keys are steel (usually mild steel or possibly wrought)..... using heat does two things..... it helps "burn" the crude, rust, etc. and the cast will "soak" the heat up more so then the steel/wrought key..... be mindful of the heat! DO NOT get things hot enough to change the color.... if you do, and make a mis-hit with the hammer, you can easily chip/break the cast head.
On a specific 50lb LG I rebuilt, I had to heat the key/head area with a rosebud, let it cool, then soaked it down with WD-40 several times, over several days before the key finally broke loose. Again, patience is mandatory here! Trust me, getting in a hurry with tearing down these hammers does nothing but cost you money and creates grief.
