how do you remove costumes?

I don't think you can get rid of the costumes. And, believe me, you don't want to. When you do that, future parties and yeti lunchboxes will start giving those again i s'pose. Ince all your costumes are obtained, all you'll get are useful items.

well an option to toggle them on and off wouldn't hurt, you'd still have the costumes so wouldn't keep getting the same ones but wouldn't have to put up with their awful appearances