If you have shared an Evidence with another organisation, you can revoke that access at any time. Revoking removes the organisation's visibility of the Evidence immediately.
Revoke shared access
Go to Evidence Tracking > Evidences and open the Evidence.
Select the Sharing tab to see the organisations (Trusted Parties) it is shared with.
Find the organisation you want to remove and use the Actions on its row to remove access (or to enable/disable access).
Note: revoking takes effect immediately.
To share an Evidence in the first place, see How do I share an Evidence?