Spray ammonia-based window cleaner onto the glass insert on the pellet stove. Completely cover the melted plastic. Wait a couple of minutes for the window cleaner to penetrate the holes in the plastic and to seep under its edges.
Hold the razor scraper so your thumb is on one side and your fingers are on the other side. Extend the blade. Rest the blade on the glass so it forms a shallow angle, almost parallel with the glass insert. Apply light to moderate pressure as you slide the scraper across the glass, lifting the melted plastic as you scrape.
Wipe the blade clean with a soft rag. Continue scraping the melted plastic from the stove. The plastic is easiest to remove when it and the glass are wet. If necessary, reapply window cleaning solution to finish the job.