You can disconnect from BigCommerce in two quick steps.
Disconnecting your BigCommerce account from Katana:
Go to the Integration Marketplace and find BigCommerce under the Connected Integrations area.
Click on Manage, and choose Disconnect from the dropdown. A popup box will appear where you'll need to choose Proceed to BigCommerce.
From this screen, you can uninstall the app from BigCommerce.
Once disconnected, 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.