You can spray some pestcontrol solution in the attic and walls. Usually bats are seen in the nights. Switch on the lights in the attic during the nights. You can keep some pest control machines which have a high frequency sound that can keep away bats.
Seek specialist help in particular some species of bat are protected. Also avaliable are some high frequency sound machines used for pest control, these are inaudible to humans but cause disorientation in bats.