To prevent squirrels from entering your property, you must 1) discover what is attracting them in the first place, 2) find out how they are making their way in, and 3) take the necessary steps to ensure they cannot enter again.
Why Squirrels May Enter Your House
How Squirrels May Enter Your Home
Some things that may allow squirrels into your home include:
If you notice squirrels entering your property, observe the animals in order to figure out what is attracting them. After you have determined the cause, remove it.