Coving?
A wide flexible stopper blade (like a putty knife but thinner, wider (and straight)
Go along one glued edge .. then the other opposite edge.
Never use the damn stuff again.
Only use plaster coving (long lengths as possible) if you absolutely have to use it.
you'd do better and faster with a wallpaper scraper. I put up with wallpaper paste it will be easy. If put up with special cement it'll be harder, and you may need a heat gun to soften it prior to scraping off.
I'd think twice about heating anything that has ... 'styrene'... in it's name.
You are looking for trouble, as it will give off toxic fumes if it starts to melt.
Duh, I didn't say the polysterene, I said the cement used to stick it. It sets like lumps of hard chewing gum, and a bit of heat to warm it might be enough to loosen it enough to be scraped off.