Instructions
Launch "Dwarf Fortress" and press "Enter" on "Continue Playing."
Press "N" to open the Nobles screen, as the mayor is a noble. By default, the mayor will be highlighted when you open the screen.
Press "R" while highlighting the mayor to open a list of dwarfs who can replace the current mayor.
Press "Enter" on any dwarf to make him the new mayor of your fortress, canceling the previous mayor's ban. You can now continue processing your hematite into iron bars.