Some installers use Border Adhesive regardless of the surface their hanging over. This stuff, once fully cured, is not water soluable. In fact after 20 years of this trade I have yet to find anything that will desolve it.
Try this expirement:
On a small section of the border, sand it with 100 grit paper, then wet the surface with a sponge or a spray bottle. Then apply a folded paper toweling over the wet area and then wet the paper towel. Allow the wet toweling to sit for at least 15 minutes, periodically rewetting it.
If that loosens the sanded area, then go ahead and sand the rest of the border and work at keeping it wet (no need to do the paper toweling trick) with a sponge or sprayer, periodically testing the area you started with to see if it's peelable yet.
If your border is a "Peelable" paper, the facing should peel off, leaving the backing paper in place. That backing need only be wetted and it should release from the adhsive. Sanding or scoring a "peelable" paper only makes it harder to remove. Sometimes it takes a while to get the "knack" of peeling the facing off and that leaves you peeling small sections at a time. Once you get the feel of it you can get whole sections at once.
If your border was applied with border adhesive, all I can suggest is keeping it wet and use elbow grease and perserverance. Once you get all the border off, don't try to remove the adhesive, apply several THIN layers of joint compound to smooth it off, then prime it with a good latex primer and go from there.