Bats are good unless they carry rabies! They do leave a mess below where they live -> fecal matter.
Locally some fungus has been killing the bats.
http://www.milforddailynews.com/home...-wipe-out-bats
People are scared of bats because they feel they will get tangled up in their hair! We use to fish at night up in VT and had the bats eating the mosquitos around us. We never had one run into us. They were so close you could feel the breeze from their wings as they flew past. Pretty cool!
Try using a water hose or a long thin stick to poke them out of the gap. Use a bright flash light and you can see them hiding. They can fit in a very narrow gap - 1/2" or so. Staple screen over area like Cheferson said, or put up some wood scrap or molding to cover up the opening. Better not to kill them IMHO - bats are our friends!