Try a bar of soap.
At work we're signed up to Microsoft Select so we get pretty much all their software. Part of the agreement is that we destroy old CDs. So we used to have piles and piles of them to dispose of. Some of them went into the office microwave.
Nice light display and the shiny metal in the disk turns all spider webby. Unfortunately I believe it also produce toxic fumes. Even with the windows open and fans blowing they stunk out the office and gave me a head ache.
You also wouldn't want to use the microwave for food afterwards.