If by that you mean mark them for death on sight and not physically block people from reentering, then yes.
A mayor or king is well within his/her rights to ask any person in a town, whether or not a citizen, to leave. If the person refuses to leave, he may be killed. Generally, the banished person is allowed to remove their house materials and take them, if he owns a house in the city, but depending on the crime's worth in terms of value this right might be waived.