You can disconnect from BigCommerce in two quick steps.
Disconnecting your BigCommerce account from Katana:
Go to the Settings screen and click on the Integrations tab.
Find the BigCommerce logo and click Disconnect. You'll then be asked if you would like to proceed to the BigCommerce store to disconnect Bigcommerce from Katana.
From this screen, you can uninstall the app from BigCommerce.
Once disconnected, your Sales orders will no longer be pulled from your BigCommerce account to Katana. You can reconnect your store or connect to a new BigCommerce store at any time by following the process described here.