Light a fire at the bottom of the can. Eventually it will heat to the point it explodes. And if the explosion or shrapnel doesnt kill the spider you have now just aersolized an entire can of spider killer to fill the whole room.
Interestingly enough, there's usually a list down the side of the can saying what the spray kills, but with spiders it depends on the type, with some of the nastier spiders it just irritates them
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