Are you actually planning to carry out a separation? If so, what do you hope to achieve? Acetone has a boiling point of 56degsC, whereas the melting point of Styrofoam is 240 degsC. This means the acetone would evaporate off leaving the Polystyrene at low temperatures. However, acetone is also highly flammable, so a heated distillation should only be attempted by experienced professionals with suitable equipment. If you just leave the dish in a dry place for a few days, not in direct sunlight, the acetone will evaporate off, but even that should be properly risk assessed as there would still be a fire risk.