Assuming there is not a screw holding it on (your experience with the other side tells you this), just give it a solid smack with a hammer. When I worked for Mercedes, it was not uncommon to need a 2-lb sledge to knock the disc loose. We would use two hands and swing really hard sometimes.. never heard of a messed up bearing from that.
Of course, this assumes you aren't reusing the disc.
If you aren't pressed for time, you can try spraying some penetrating oil around the hub and let it sit for awhile. It's questionable whether this will really penetrate far enough to help.
When you are putting on the new disc, use some anti-seize around the hub to help prevent the same issue in the future.
Edit for your update: Don't be shy. Smack the heck out of the rotor from the front and the back (if you can, sometimes the back is covered by a shield). Hitting it on the front face may seem counterintuitive, but it may jar it the right way to get it loose. Also, carefully aim for the outside edges of the disc. This applies more leverage compared to the center hub, which is where it's seized.
Use a real hammer.. a 2 -5 lb mini-sledge. A regular claw hammer isn't going to cut it. Swing that sucker with both arms like you are batting in baseball.
Double-check for a screw in the center area around the hub. Some manufacturers use a screw to hold the rotor in place while they build the car. The other side may have been missing this screw for some reason.
Safety Note: Wear safety glasses! You should be doing this anyways, but in particular, when you hammer at the edges of a rotor, bits of metal will come flying off.