You can use the Client - Security to limit complete access to a specific client. Refer to Client Security.
When Client Security is enabled for a client, only an FYI administrator or the users who have been given access will see any information about the client. Any other users will not see the client in the Clients List or in any other lists, and it will not show in any searches. All information is hidden from all users except those who have been given specific access. Only an FYI administrator or one of the users who has been given access to the client can disable security.
You could also set up a specific Cabinet for just this client, and only allow access to that Cabinet for specific users in the practice. You then set this Cabinet in the filing defaults for the client. This limits the documents to specific user, but the client will still display in the Clients List