In the absence of a solvent that removes it, you are left with scraping or sanding. If it is hard, you could (carefully) sand it with an orbital sander. Have you tried a Dremel Multi-Max with a flexible scraper blade? Or if it is soft on the wall, you could soften it even more using a heat gun and then remove with a scraper.

You will probably have to skim coat that part of the wall when you are done to get the wall smooth again.

It probably doesn't matter what you use really. I would use a green scrubbie pad and whatever tool you choose that gets the wall wet. I would just use a squirt bottle with hot water. This is a fairly normal experience when someone has used sizing on drywall actually. My theory is that the sizing combines with the residual paste that is left on the wall and creates this rubberized goo. This is why we don't use sizing on drywall, but I know for years it's what everyone working in the paint stores was telling us this is the proper thing to use. From now on, prime!

Use some "Chomp"-brand concentrate, & the hottest water you can comfortably stand. When you get a couple saturating applications on (~15min. apart)....

Press some light painter's-plastic or Saran-wrap into this area.
Wait a couple HOURS...maybe even overnight.

As PG said, use a Scotchbrite scrub-pad with a handle to help chew through the goo! Light-to-medium pressure. Take only 3'-4' of plastic down at a time, uncovering as you clean each section.

Faron

Sand it and or scrape it off( paste or adhesive) with a wall paper razor scraper. Or just skim over it.
Also you will need to prime with Gardz or an oil primer after you are done.

This is border. How many lineal feet and what's the height?

You're working much too hard on it.

So, what would you do, brushworks?

In some ways, this is worse than one bad wall. It's probably on all the walls of the room. You are working at the ceiling so you are on a ladder. You can't cut out the 8-9 inches of drywall. You have to make it smooth with the drywall below it. Short of applying more border, which isn't in style, what do you do?

How many feet and what's the height?

Assume that the height is the top 9 inches of walls 8 feet high. Assume that the room is square and each side is 10 feet.