There are no safe yet effective ways to kill bats en masse. If a bat roost is an issue, it is better and usually easier to use netting over the holes to get them to leave permanently. In most jurisdictions, killing bats is illegal, and those who attempt to kill bats may be hit with very large fines.
Bats eat live insects and are adept at catching them in the air, so attempts to get them to ingest poisoned insects are usually fruitless. Aerosol-based poisons are equally as ineffective because bats can fly out of the area rapidly.
Unlike insects, bats are mammals, and poisons that affect small mammals may affect humans as well. A bat-lethal quantity of DDT, for example, would also pose a hazard to the family inside the home. In addition, bats have large muscles that break down when they die, causing an offensive odor. If those bats die inside the house or in the walls, the odor may linger.
Removing bats typically involves putting up a piece of netting that is shaped like a windsock around the holes through which they enter and exit. This netting allows them to exit but not enter.
Bats are generally harmless to humans, and they eat a lot of insects — up to 8,000 per night. Bats often eat mosquitoes, which are linked with malaria, heartworms and West Nile fever.