Depends if you are saving the blank or not.
If saving the centerband and not the blank, then remount the blank, and part away the wood up to the centerband. you can undercut if you have a small hook shaped tool, but usually you will not need to. by this time, the glue line is usually small enough to be able to break by hand.
If you are saving the blank, and not the centerband, then you can start to "worry" the centerband off with a parting tool, by remounting the blank and running the lathe at a moderate speed, and trimming through the MIDDLE of the centerband. Usually, at some point, the glue line will give way, the centerband will start to spin on the blank, and you can then pull it off. Use an older parting tool for this, and not a good one, unless you do no mind re-sharpening your parting tool a few times.
If you are saving the blank and the centerband, you can once again re-mount it on the lathe, and worry if off with a SHARP razor knife right at the joint between the blank and the centerband. Once a small space starts to appear, it then becomes easy to get the centerband off by hand.
Hope this helps
Jerry