Most of us might find bats intriguing when we see them fluttering around outside in the dark. Bats can even be beneficial, since they will keep mosquitoes at bay, eating as many as 8000 on an average night. You might not be as thrilled about having them as your tenants however. Bats that make a home in your attic can make quite a mess with their droppings, and cleaning it up could expose you to disease. They could also be a nuisance when you are trying to put things in the attic and they are disturbed and fly around.
There is a humane way to deny them entry into the attic. You would need to find their entry point and then install a one-way excursion device over it. This will only allow bats to leave the building, not get back in. Of course you will need to make sure there are no other points of entry for them to use.
It is also possible to make the attic inhospitable for them by installing lights in the attic. This will make it less likely for them to choose your attic as a sleeping place, since they are nocturnal.