To disconnect Xero from a Scenario and remove all the Xero data it contains, select the Xero icon to the right of the Scenario name. It will bring up a menu that shows all the Scenario Import Settings and date ranges.
Simply click on Disconnect in red and hit OK when prompted. All of your Xero data and settings will be removed.
Adjusting your import settings is an alternative to disconnecting that provides you with more options. If you would like to disconnect Xero going forward but leave your previously imported data in the Scenario, see Changing Import Settings in Xero
Note: Be aware that any manually created items that are in a Category that has been imported from Xero will be deleted along with any information that has been imported into that Scenario or Forecast. Make sure you have moved any manual items into a separate, manually created Category.