1) Go to the Admin CRM module (top menu -> Admin CRM).
2) Click on the tab "1. User list".
3) Click on the user ID of the concerned user.
4) The user file opens. Click on the "Deactivate" button. Confirm the confirmation.
5) The user is disabled.
From that moment on, the user no longer has access to Simple CRM. The user's iCal feed is also disabled.