Have you tried the e-mail login?
Cpanel will automatically make you an e-mail account with your cpanel username and password, so to check, make an e-mail that is set, for example. [email protected]
with password of password.
Your login will be [email protected]
then your password will be password. That will allow you to login to your e-mail and you will not be able to move through anyother part of cpanel. Notice that your login name is your e-mail not just the first part, that is how it is supposed to work. hope this helps you.
If you enter your cpanel info, yes you will be given access but that is not what you need.
Have your clients follow the same basic setup as I just sudjested, and then you will have no issues.