Classic solution is to find out how they're getting into the attic, wait until they're all out hunting ca midnight or so & plug the hole (or holes) so they can't get back in.

If ya don't get 'em all first time round, you may have to re-open the hole & let the remainder out a second night & then plug it up again.

Professional wildlife control dudes ( e.g. "VexCon" of "Dirty Jobs" fame) have one-way devices they can install to let 'em out & not back in.

You don't want to be killing 'em if they're a protected species - which a lot of 'em are- especially with the current mass bat die-offs in the US due to White Nose Syndrome. see http://www.fws.gov/whitenosesyndrome/