I'd suggest going with Younguy's advice, above. If you pop a squirrel with a .22 shotshell, he probably isn't going to expire on-the-spot; he's going to get into some deep, dark place in your attic, THEN die, and stink up the place for the next few years. Same thing if you just seal-up the hole; one may still be up there, and could make a mess before AND after he dies. If you trap them, then you really KNOW you removed them from the attic.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)