It's easy to do and important for Election Integrity.
Some people are still on the rolls because the US Post Office doesn't always report your move to the Board of Elections. You may not be removed from the voter rolls for a long time, creating inaccurate voter rolls. Some house hold members may have been removed while other are still on the rolls. If that's the case, you have to do it yourself.
The Union County Board of Elections will remove you from the county's list of registered voters if you complete the Cancellation of Voter Registration Form.