If they are really stuck, and the hammer won't do it, you can try this:

Start the lug bolts for each wheel, take them in by hand until they just touch the wheel.

Back each lug bolt out 1/2 to 1 turn.

Let the vehicle down.

If these are only the front wheels, try turning the steering wheel from side to side a few times, this may do the trick. If not, or if you're dealing with the rear wheels also...

Drive the vehicle slowly back and forth, not far, just in and out of your garage, driveway, shop, or wherever. If you can go over the small bumps from street to driveway, all the better.

This will usually loosen up very stuck wheels. Obviously you don't want to get carried away and go out autocrossing with loose lug bolts, but a little back and forth and side to side won't hurt. the wheels center and ride on the center hub, and this is where corrosion forms. This also ensures that very little, if any, actual movement will occur when the wheels are broken free with the above method.

MV