How do I disconnect a source?

Settings, Integrations, then Disconnect on the source. Access ends immediately and the stored token is deleted. For Shopify you can also uninstall the app from your own admin, which revokes access from Shopify's side with the same effect.

Historical data already synced stays in your account until you delete it, so your past P&L does not empty out when you disconnect an ad account. Reconnecting later picks up from where the data ends.