Ground hogs under the house

Well, we have just discovered the a family of ground hogs have taken up residence in our crawl space. We believe that they came in through a hole in the brick, but have since made multiple entrances. We've seen about six or so. They have torn up the duct work that is under the house (and we'd been wondering why the air conditioner wasn't cooling the house!!).

Anyway, thought you all might have some good ideas on how we can rid ourselves of these uninvited guests. Does anyone have a way to close up the holes and keep them from creating news ones?

thanks in advance!