Spandex is a polyester-based fabric that's essentially a form of plastic. Spandex material, like nearly all plastics and rubbers, can be manipulated and shrunk using heat. If you come across a spandex hat that is a little too big or stretched out, shrink it back into usable condition with a simple process.
Set your washing machine temperature on hot. Protect the sheen of the fabric by placing the hat in a pillowcase or delicates bag.
Wash the hat on hot for a full cycle.
Immediately remove the spandex hat from the washing machine. Set your dryer on high, adjusting the duration for about an hour.
Remove the hat from the dryer. Allow a few minutes for the hat to cool. Test the fit, repeating the process as necessary.
Ironing the hat with an ironing cloth might help expedite the shrinking process.