Depends on how big the hole is they're getting in at...I put screen over the opening they were using with a undercutting bend so they could flop out but couldn't get back under to get in. No more bats.
Watch in the evening to see where they are leaving. Then, when they are out of the atic, plug or cover the hole so they can't get back in. Simple if you have only one access/egress hole, but might take more than one application if you have several holes.
Not sure if it's federal or Colorado law, but bats are protected here and exterminators won't touch them. Installing bat houses and plugging the access holes with steel wool worked for us.
When I had a bat in my house. I just closed it all up and waited. My little fuzzy serial killer (20lb cat) took care of it. Had the furball checked out and he was fine. Just flying mice.
Then went over every inch outside and plugged any holes I found. Any holes, they can get in some very small openings.
Just dealt with this myself. They were nesting in my eve vents in the shop. I read that they don't like mint. My wife had planted some mint in our flower beds so I pinched off a branch and hung it on the inside of the screen. Gone ever since.
Hang some bat houses in nearby trees so they have somewhere to live.. They eat an insane amount of bugs, especially mosquitos, so they are good to have around.
The exterminators around here put a net around the opening to catch the bats as they exit, fix the hole and then after payment release the bats. Then the neighbors get bats. They charge an insane amount of money to do this.
Fill a pressure vessel like a 5 gallon fire extinguisher with warm soapy water and give them a bath. They hate that and will leave immediately then close up the hole they left through and clean up their ****. They will go live somewhere else, clean but unharmed.