Both of the former ideas are...well....bad advice.
Not all bats leave every night to hunt for food. Likewise, many retun periodically throughout the night. Finally, looking at the post's date, you were right in the middle of brooding season and would have orphaned any young that may have been present. It's not only a matter of ethics, but think about it. A bunch of dead baby bats in your attic and walls. MMMMM...what a smell, not to mention the insect issue it would cause.
As far a ultra-sonic devices go, worthless too. Studies have proven these to be ineffective and more than one manufacturer had been shut down due to boasting false information regarding their success. (I have photos of these devices with 3 inches of bat droppings piled on them, that ought to tell you how the bats feel about them).
Bats can only be removed effectivly using an eviction/exclusion combination. It must be done correctly. Simply "boarding up the hole" will only prompt the bats to find the next easiest way in, and trust me, they will.
Call in a professional or if you would like to do it yourself go to Batcon.org for more detailed information.
Animal invaders are never as cut and dry as most would assume.