[Dovecot] Excluding some system users after configuration

Dear readers,

Consider an installed Postfix configuration (2.3.3) on CentOS. Now I want to install Dovecot (alas only version 1.0.7.7 is available) to provide IMAP services for a few system users, but not all. Most importantly the mail delivery by postfix for some of these system user /must/ not change after the installation of Dovecot.

How can I prevent the installation of Dovecot to touch some specific system user accounts?

Thanks in advance,

/Twan