Spray foam. There are often large gaps around the drain pipes and vents. It will keep the spiders and other stuff out too. I had large gaps around drain pull rods, sliding electric conduit to the slide. I greased the rod surface and foamed around them and the foam doesn't stick. It allows the rods and tubes to slide in the foam after it dries.
Check for gaps underneath and in the basement and closets and cupboards. Vent pipes will run from the tanks up and often have large gaps. After looking at mine the first time it wasn't a matter of mice, squirrels could easily have gotten in.