Create Client without Sites option

Just wanted clarification on how clients should be setup and how to restrict their options.

So far I have created a client for each email domain we host, we don't do websites, and have assigned these to the appropriate domains under the Mail Domain settings. I have also assigned these to the DNS zones we have configured for each client.

Logging onto the client account shows tabs for Sites, DNS etc which I don't want to appear. To get rid of these tabs for the user do I simply go into system > CP User and edit the module list for the username automatically created when adding the client?

If so what does the following message apply to?

"WARNING: Do not edit or modify any user settings here. Use the Client- and Reseller settings in the Client module instead. Modifying or changing Users or groups here may cause data loss!"

ie if I disable the DNS module because I don't want the individual client to make changes here will that delete the current records I have created where the client has been specified. If this is the case I assume I would have to un-assign these from the client and then make the changes under CP User. Or will it simply remove the tab for the user but keep the record so that I can make changes as and when required.

Basically I only want to give clients the option to manage their own email addresses and email quotas.

Many thanks for the help in advance.