Today I am in Florida giving a seminar to a group of Federal judges on the law and economics of Federalism and Crime. One of the surprising things that I discovered in my research is that cities, counties, and even most states can legally banish criminals from their borders. I say most states because, for example, the Georgia state constitution makes banishment illegal. Georgia judges, however, have found a way around the law they have imposed "158-county" banishment. (If you guessed that Georgia has 159 counties give yourself two points.)
Banishment is a particulary noteworthy example of a negative spillover – banishment benefits the state doing the banishing but only at the expense of other states. I will suggest to the Federal judges, therefore, that state banishment should be illegal.
There are some arguments for banishment from a city or county. Banishment, for example, can remove a criminal from negative peer influences. Whether the advantages outweigh the spillovers is an open question but city and county banishment should be left to the states because the state government can internalize the city/county spillover.