You need to be able to seal up any cracks the bat may have used to get in.
Open a window before dusk. The bat should go out to feed. When it's out shut
the window, make sure all cracks are sealed. That might do it. BTW there are
companies that specialize in "Bat Exclusion". You might look some up for
more info.
Good Luck - they are delicate creatures yes?
Susan