You probably aren't going to get it off. It melts the surrounding abs and fuses the abs plastic and the cement together.
But when you tried to sanding you only used 320 grit. Thats considered fine for wood, but is very course for plastic. Look in the automotive section of most department stored and get a variety of auto sandpapers. Try sanding a scrap piece with 400 then 800, then 1500 and polish it with novus. You shouldn't see the scratches then. If you do use some 2000 grit sandpaper and re-polish.