Once a church champion pairing has been set up, removing it is very straightforward.
From the church drop down, select the top option "No longer a champion" and click "Update".
The page will reload with that pairing remove. If that was the only volunteer - church pairing for that volunteer then their church champion role will automatically removed. If they have multiple pairings then you will need to repeat the process for any other churches you no longer want them to coordinate.
A note will be added to the volunteer record for each pairing that is removed.