Smoke them out with ammonia.

“ There are scents that act as repellents to opossums, just as with many other animals. The smells of ammonia and mothballs are intolerable to them. A great method of using ammonia is to pour some into a can and use a long piece of cloth to act as a wick. The ammonia will soak the cloth and the fumes will travel through the air. If you are going to use this method, it should be done wherever a possum may hide. Toss some mothballs around the area for good measure as well. ”

Source: http://wildlifeanimalcontrol.com/opossumrepellent.html