When a staff member leaves your trust, or if you wish to disable their access for any other reason, you can do this by going to the People focus area at the top of the screen:

From here, click on the name of the relevant staff member, then click the System Access dropdown on the left, followed by 'User Accesses':

When you click Remove Access in the Actions column, that user will no longer be able to log in to the trust.

If you need to reinstate their access at a later date, you can do so by going to the Invitations section, and sending out a new invite.

Thanks for reading!