If "decorate" means to make something more attractive:
* Strip: to remove decorations, often used for furniture or rooms.
* Deprive: to remove something valuable or appealing, suggesting a loss of decoration.
* Desolate: to make a place barren and unattractive.
If "decorate" means to add embellishments:
* Simplify: to make something less complex or ornate.
* Plain: to make something basic and without any embellishments.
* Depersonalize: to remove any personal touches or decorations.
If "decorate" means to add something to make it more appealing:
* Disfigure: to damage or make something less attractive.
* Mar: to damage the beauty or perfection of something.
* Degrade: to lower the quality or value of something.
The best antonym will depend on the specific situation you are referring to.