What can I do to get rid of an underground nest other than using poison?

Wait until all the insects are inside the underground nest at night. Put a bucket or large bowl over the opening and mound dirt around the bottom rim of the bucket. Check the area the next day to see if the yellow jackets have found another exit from their nest. Wait until night and cover that hole as well. This works because yellow jackets do not excavate their own nests, they use existing holes left behind by gophers or mice. Once you have the holes covered they will die inside the nest. It's up to you when you feel like enough time has passed to remove the bucket, but I wait a couple of weeks if not longer.