SPIDERS coming in thru AC.....Help

I doubt that bugs come in from the roof. The AC is sealed on the outside (otherwise water would leak in during a rain). The only spider that I worry about is the Brown Recluse, and they are unlikely to be seen crawling on a ceiling. But if you want to get rid of spiders in the AC, you should be able to remove the inside cover and spray with some insecticide. But allow plenty of time for it to evaporate before running the AC again.