Let loose some bigger spiders to eat the little ones?
Praying Mantis or birds might work too.
Do you like the smell of peanut butter? You could mount some of those mouse sticky traps around it, so the spiders can't cross to get to the detector, or some double sided foam tape inside it, around the sensor.
Is there an LED always lit on it? Light can attract other insects, which attracts spiders.
Are there a lot of spiders there?
I've never had this happen to mine, nor have I heard about it from anyone I know IRL. Maybe it's just a one-time event that will never happen again.