I'm not aware of any plates on the market to cover the hole. However...
24252(a)V.C. wrote:
(h) Any supplemental stoplamp installed after January 1, 1987, shall comply with Federal Motor Vehicle Safety Standard No. 108 (49 C.F.R. 571.108). Any vehicle equipped with a stoplamp which complies with the federal motor vehicle safety standards applicable to that make and model vehicle shall conform to that applicable safety standard unless modified to comply with the federal motor vehicle safety standard designated in this subdivision.
A deleted or non-functional 3rd brake lamp is reason enough to be pulled over and cited, according to the California Vehicle Code. Unless the spoiler has a brake lamp on it, you might want to pay attention to this as a heads up!