Hi Robert,
Actually, I posted this in General as well, and had a great reply from Luis who said:
"Turn on the "Prevent account creation when authenticating" feature under Site administration / Plugins / Authentication / Manage authentication. That will prevent Moodle from creating an account for those 90 users when/if they try to login and it will effectively block them from the site. But you'll need to make sure the 10 users who you do want to be able to login already have an account in Moodle, else they'll also be blocked.
If the 90 users already have an account in Moodle you'll need to delete or disable their Moodle accounts."
...which has solved the problem for me. Turns out, below the table of plugins there are many more options on the authentication>plugins>manage authentication page - which are easily missed. : )
Might work for you too!
Hugh.