You should be able to unscrew the flange off the waste arm. Use long nose pliers. If it doesn't budge, cut it off, but don't damage the tub.
You should have access to the plumbing from the other side of the wall. Most of the times there is a small access door. If there is no such door, you'll need to cut some drywall.
Once you gain access, you can also cut the waste arm off and the flange will come out.
This job requires experience.