Take a pan of feed corn soak it in beer for 24 hrs, put it out where they like to hang out, just a few hours before sunset, let them eat it and when they fly back up to your roof wait for the sound of drunken stumbling pigeons and then when they fall off the roof finish them off any way you like.
I heard this method from a very old fella and he was sure convincing on how effective this was.
I've used bb gun myself and they don't seem to hang out at my house anymore.