Getting melted Tupperware and Play-Doh out of an oven
Most household plastic will become more brittle the colder it is; Tupperware is likely the same.  

You could try putting ice cubes on one of the thinner parts and see if it becomes brittle enough to snap.  If it does, you could try freezing the whole pile with dry ice.  Other freezing techniques include squirting a can of compressed-air dusting spray held upside down.  See for example dusting spray.  Or if you need more volume, a CO2 fire extinguisher.  Spending $30 on an extinguisher is still cheaper than a new oven.

Wear thick leather gloves when trying any of the freezing techniques.