A well-fed possum can’t be scared away by anything you could do, no matter how loud or how long you smell. They mostly only leave the area when there's something that is a lot more dangerous than humans - such as a coyote, or when it's getting dark outside.
This doesn't mean that there aren't other methods that may work, it is just that you might want to hire a professional if you find yourself dealing with a truly settled in possum that simply holds its ground due to being well-fed and comfortable.