While bats are beneficial to have around, as with any animal, you really do not want to share your living quarters with them.  The guano or bat droppings have a strong pungent odor.

First, keep in mind that while bats living in your home are not welcome, having them in the neighbor hood to control night insects is a huge benefit.  So, while removing them, you may opt to have a new home ready for them, whether it is a bat house near their current entrance to your house, or a nearby tree or under a deck or awning.  Some people will actually collect the bat droppings or guano for fertilizer, it does have a strong odor. 

Another concern among bat experts is the white-nosed syndrome, potentially killing many bats across the United States.  These beneficial animals are important to protect, there is no good reason to evict them during the "young" season, or try to poison them....here is a better alternative...

Evict bats after the 2nd week of November, when the young can all fly, and before June, when they start the next generation of young, any earlier, and you will run the risk of killing the young, which not only reduces that generations number of bats, but also will cause unnecessary odor in your house or dwelling.  

Before you start evicting the bats, determine if you want to keep them around...I and many others would strongly recommend keeping them.  You may not want to attach a new bat house to your house, but consider a nearby tree or 4X4 post.  We sell a few different sizes of bat houses at Barn in the Sticks, and several different designs.  Install the bat house before you begin eviction. 

Next, watch the bats exit your house, attic, or shed at night, (usually on the western side of the building), and seal all other exits they are not using, so they don't start using them when you begin eviction.  Use a permanent fix, not expanding insulation, but caulk, trim boards or metal.  Next, install a bat excluder, a device designed to allow bats to exit easily, but due to how it is constructed, they cannot reenter.  The ones sold at Barn in the Sticks use a 5" piece of PVC pipe, so the bats can exit, but not reenter due to the slipery interior walls on PVC piping.  We carry two models of bat excluders, the first is for a horizontal mount, like your soffet, the second is for a vertical mount, like on the side of your house.  Then tube needs to point towards the ground in order to work.

Continue to observe the bat excluders at night, as well as other areas of your dwelling, to ensure they did not find another entry.  After you no longer see bats exiting your building wait one to two more weeks to ensure they are all out, then remove the bat excluder and permanently seal the hole.  

The next step is up to you, you can either chose to keep the bats around or remove them completely.  Bats are a beneficial animal to have around, especially to control the nighttime insects in your yard, like mosquitoes.  Bats will generally eat their own weight in insects every night. 

Should you decide to keep the bats, you can purchase or make a bat box, and place it near the bat excluder you placed on your house.  Some of the bat houses sold at Barn in the Sticks hold up to 600 mosquito eating bats.  Others, like the Barn in the Sticks model fit over an existing vent in your attic, so that the bat is near your house and at the same time, is not IN your house.  Whichever you decide, the closer you put the bat house to the bat excluder, the better the chances are of the bats finding their new home.