How do I keep mice out of my pool heater in the winter ?

The mice made so much of a mess it's taking me more than 3 weeks to fix it.How do I keep mice out of my pool heater in the winter ?
Line the box with hardware cloth, its basically a screen type cloth, made of metal. They cannot chew through it. Also, put moth balls around the heater box, the smell keeps out most vermin.How do I keep mice out of my pool heater in the winter ?
Moth balls
take a shot gun and kill them
use alka seltzer they blow up in the insides
Mouse Away Pouch See this site. They are reasonably priced.
What?!! Oh that poor mouse! Aww!! I'm sorry sweetie but it's so sad!! *starting to cry*

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