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Spiders can fit through small places so there's not much to do to keep it out if it wants in. If you're in the northern hemisphere, the weather is getting colder, which is going to push spiders and insects to seek warm shelter (your house). They'll probably prefer your garage (if it's warm enough) because there are more places to hide. You can buy sprays at your local garden center that provide a barrier around the house. I would spray around the outside outline of your doors and windows, and a good patch in front of each door, and around the garage door. I would also spray plant-friendly pesticide around the entire perimeter of your house, especially if you have flower beds or plants of any kind against your house. This should be a regular practice to prevent any bugs from entering. If you don't want to use pesticides, a solution of vinegar and water should work decently. (Not on plants though, it messes with the soil pH)