Natural, chemical-free bat repellent methods are wall repair, aluminum foil, mirrors and Mylar balloons. These methods keep bats from returning to homes and nests made within residences.
Aluminum foil and mirrors have a shiny surface, creating reflective light in small spaces, deterring bats. Aluminum foil creates noise when moved, scaring the bats. Mylar balloons have a shiny surface as well, which combined with the movement of a balloon chases bats away.
A definite way to make sure bats cannot return to an area is to seal off the entry points they are using with an outdoor caulking sealant or thick plaster.