In the portal, click the 'Invite Clients' button and then 'Manage Invites' (1). In the next panel, switch to 'Portal' (2). There will be a list of portals for that client - untick the one you want to disable (3). You can tick it to re-enable it later if you want.
Alternatively, you can disable access for individuals on the 'Access' page of the panel.